Large disparities in lifetimes between characters can be problematic, especially if their relationship goes into the romantic field. This plays on the very mundane fact a character will probably be long dead before another is, and the emotional issues it raises. How much this is alluded to is usually related to how concise the story which addresses it is. The most obvious way to address the issue is to even out the age-bracket on either end. In older fiction, characters may be granted immortality, but recently the reverse, making the immortal mortal, is more common.
For the Half-Human Hybrid progeny of these pairings, it's even harder to find a compatible mate lifespan-wise, because generally their lifespan is some sort of average of their parents'. They'll live longer than the mayfly, and shorter than the immortal. Such individuals tend to be rare, so there's no option with "just the right amount of lifespan."
If the immortal is a vampire, they have an obvious way around this. Although this has a tendency to piss off the would be mortal, or it turns them evil, as opposed to the Friendly Neighborhood Vampire the original was.
Can raise Fridge Logic issues in an Anyone Can Die franchise with a high body count, given how few characters in such series even get the chance to live out a full lifespan.
Anime is known for its treatment of the subject due to the Japanese concept of mono no aware; roughly, the beauty of transient things.
I Hate You, Vampire Dad may be the result. May-December Romance is the more mundane version without the mayflies. Compare Reincarnation Romance. Contrast Eternal Love, where both partners are immortal or at least long-lived.
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Anime and Manga
One of the main plot elements of Crest of the Stars. There is greater focus on this in the manga.
Frequently suspected to be a touchy subject in Ah! My Goddess, the matter is only obliquely referred to in original manga and an episode of the recent show featuring a ghost. This is tricky to pin down though, as we've seen the ladies at different ages for different reasons (flashbacks to Heaven, magical effects, and so on) but it's a matter of canon that the goddesses' Earthly bodies are technically projections of the universal computer Yggdrasil. Keiichi is very aware that he is destined to be with Belldandy for the rest of his life, not hers. Interestingly, he asked Peorth about this (in one of the later Manga arcs); he couldn't bear to ask Bell. She fondly answered that they never forget the people that they have met over the ages and cherish them all. In the original Japanese, it was implied that it was Keiichi's innate acceptance and understanding of their relative differences that was part of the attraction he presented to Belldandy and the other goddesses.
Unusually, Yokohama Kaidashi Kikou deals with this in a fairly straightforward way: despite her obvious affection for Takahiro, Alpha avoids becoming romantically involved with him because she knows she will outlive all of her human friends. She encourages Makki to pursue him, and eventually settles down with Kokone .
The trope was touched on in a side-story manga in the Record of Lodoss War series centering around the human Parn and elf Deedlit (Deedlit's Tale)`, but received practically no attention in the anime version of the story. However, it did get an english lyric song on the the third TV series OST dedicated to it: "Evermore."
Played with: Sara and Lottie in Soukou No Strain both become Reasoners to see their brothers again, because relativistic effects (Interplanetary travel means spending time near lightspeed and dealing with the attendant time dilation, since they don't have FTL travel) declares that once they return, the girls will be long dead. It doesn't work out well.
One vampire in Trinity Blood is urged to confess his attraction to a human, as humans grow old and die too quickly.
Any romance with the Edels in Elemental Gelade falls into this. Aside from a single-episode plot, this is mostly ignored.
Played with in Durarara. Celty and Shinra have come to terms with their love for each other, however the age issue is never really brought up. Though Shinra has been in love with her since he was four years old. Squick.
Averted and played with in Mnemosyne — the main character is an office lady in her early to mid 30s whose secretary and lover appears to be around half her age, if that. And then we find out the office lady is a Nigh InvulnerableBadass Normal who won't ever die, which seems like bad news for their relationship, until the secretary admits that she, too, is immortal.
Mermaid Saga. Even the one relationship that can avoid this has issues thanks to the disparity in emotional age between Yuta and Mana.
Played straight in Brigadoon: Marin and Melan, where the young Barrier Maiden was sent to earth for her safety. She's taken in by a kind Japanese family, but her sense of time is much different from theirs, and she barely ages at all before the couple die. She also marries their grandson, and she explains that she would die before him and that she, technically, wasn't human. He doesn't care and they get married anyway. Again, she seems as if she's in her twenties by the time he's lying on her death bed in at least his seventies. In the manga, at least. In the anime, the situation was somewhat different (and in ways, a lot more confusing).
In Fullmetal Alchemist, Van Hohenheim and Trisha Elric have this problem, and he is desperately searching for a way to become mortal so he can die with her. In the first anime, the problem is slightly different, because he's not so immortal after all.
In Spice And Wolf, this trope provides most of the angst Holo, a wolf goddess, feels. She's well aware that she'll live much longer than Lawrence (as shown in the OVA leading into season two as well as a couple of episodes in said season). She claims to have accepted this in season two's final episode but whether she truly has or was just being tough is up for debate. In the final light novel she marries him.
A variation of this trope appears in Dragon Ball Z; according to the ''Daizenshuu'', Saiyans have roughly the same lifespan as humans, the only difference being that they retain their youth. Vegeta and Bulma, for example, would probably die of old age around the same time, but Bulma will have aged much more visibly.
Parodied with Isaac and Miria in Baccano who finally notice that they haven't aged in over seventy years and start panicking over the prospect of outliving all of their friends...completely oblivious to the fact that said friends also haven't aged a day since 1930.
A huge concern for the fans of Kyo Kara Maoh, a show whose main plot is ending racism between humans and a race of demons that age at a bit less than a fifth the rate. Fifteen-year-old main character Yuuri is engaged to the eighty-two-year-old Wolfram, and an interracial marriage has taken place without mention of the aging discrepancy. Addressed later in the tragic romance between Cheri and Don Hiri, but Yuuri's aging still seems to have occurred to no one (though he occasionally makes old man jokes to Wolfram in the manga). Given that Yuuri is the demon king, not only in name but in occasional Alter Ego bouts of spectacular magic, it's really not clear that he has a normal human life span.
In Sailor Moon, senshi live for hundreds of years depending on version. Unless Usagi somehow gave the power to mortals, any relationship the senshi could have would end with their mate dying way earlier than they. In the manga it's stated that everyone in Crystal Tokyo becomes extremely long-lived.
In Spirited Away, this is what would have happened to Chihiro, a human, and Haku, a river spirit.
In Inuyasha, the title character is half-demon and thus is implied to live for centuries, while his love interest is a mortal human. The canon never brings up this fairly obvious problem, not even once, leaving Fanfiction to play with it in various ways.
Karin has the relationship between Karin Marker and Kenta Usui. She does realize that she will outlive him as she is a vampire and he is a human. However, after her memories of her family are erased and her First Kiss with Kenta stops her ability to produce blood, Karin now has the lifespan of a human and no longer remembers being a vampire, thus she is now capable of living her life with him normally.
Hana, the 14-year-old girl (for most of the story) of Hana To Akuma is in love with Vivi, a demon who will live centuries longer than her. Naturally, he is constantly tormented; he hates celebrating her birthday because it means she's closer to death, and frequently counts down her remaining years in panic. When he comes to return her romantic affection, he tries to leave to let her live a normal life. It's futile, though, as he can't keep away and she's more than happy to continue their relationship. They marry, and she dies at an old age, having spent their life together. It's a Bittersweet Ending though, as she left him two children who have lifespans as long as his.
Ojamajo Doremi: During the Mo~tto! season, it is revealed that this is the reason the Witch Queen from two reigns ago goes crazy and creates all kinds of troubles, such as cursing Hana-Chan and creating the Smiling Moon rulenote Travel between the human world and the witch world can only be done when a smiling moon appears in the sky. Hana-chan bringing Doremi into the witch world when the smiling moon was not present prompted the former queen to curse her., the Magic Frog Cursenote Any witch declared to be so by a human will be turned into a little green blob-like creature known as a Magic Frog., and the Cursed Forest.
Tayutama the main character expects this trope to happen to himself with his wife Mashiro only for the opposite to happen when her body is weakened too much and she's forced to sleep a century or two to regenerate effectively having her 'die' while he's young. The odd ending probably recreates the trope straight but isn't explored.
Pita Ten being an angel Misha is extremely long lived or immortal. This is only an issue in the manga but not the anime where she fell in love with Kotaro's great uncle Kotarou who commits suicide so he can spend his life with her only for that not to be the case. Kotaro is the reincarnation of Misha's love which is why she greets him at their first meeting with 'Let's go on a date!' she's waited some 80 years to see him. Since that isn't the case in the anime it makes her seem more of a Cloudcuckoolander.
In the manga Beautiful People, a vampire encounters an abandoned little girl while he's out "feeding". The little girl had apparently been abandoned for being of mixed heritage (she's a redhead with blue eyes in Japan). For reasons he doesn't understand, the vampire decides to care for her. The girl isn't perturbed by his vampiric nature; she merely feels sorry for him because he can't look at a blue sunny sky like she can, and she tells him that he just needs to look at her blue eyes to see the sky. As the decades go by, the two grow closer even as she grows older with the vampire growing weaker as well since he's abstaining from feeding on blood for her sake though she continues to offer hers for his sake. Then one day she closed her beautiful blue eyes forever. The vampire doesn't grieve because he knows how to see her again. He opens the windows to the sunny blue sky...
Actually her true form is an adult woman, but takes on that form so as the Royals don't realize she can give birth and will be forced to marry one and give birth to their child as a pure blood female vampire can only give birth once.
In Top 10, a satirical deconstruction of the superhero comic, a minor background action outside a courthouse turns this on its head. Two men feud over "The Immortal Woman", because they were extremely shallow, and wanted to have a trophy wife who would remain supermodel-perfect for their entire lives, and thus not need to be dumped and replaced after a few years.
In The DCU comic Infinity, Inc., a subplot of the "Stream of Ruthlessness" story arc involved Earth-2's Wonder Woman seeking a means to restore the youth of an aging Steve Trevor, whom she had married after World War II.
Similarly, in the late 1980s comic book mini-series Squadron Supreme, Power Princess (a pastiche of Wonder Woman) is shown caring for her septugenarian husband, whom she married in the 1940s when she first left Utopia Isle.
Joe Kelly repeated this in a JLA comic where Wonder Woman is shown taking care of an elderly and dying Bruce Wayne in a dream sequence.
The Sandman included the character of Hob Gadling, who was made immortal. In one of his stories, he states, while weeping next to the grave of one of his latest companions to die, "I thought we'd have longer. It never gets easier, people you love not being there any more." Dream's frequent though illegal romances are all of this form, since Dream as one of the Endless is as old as the universe (10 billion years for Neil Gaiman's purposes) and so even goddesses and really old witches will be outlived. Though he probably usually manages to destroy the relationship before old age becomes an issue. His brother Destruction has a similar relationship, with a goddess who only lived for a few thousand years.
In the Heroes online comic book, one issue shows the many wives of Adam Monroe (and also spoils the end of Heroes volume 2, so watch out). One of these romances even sort of works out. Adam and his wife stay together though they must pretend to be mother and son, and later, grandmother and grandson. The twist is it's revealed that he is currently married, and expects his wife to be able to rescue him from his current predicament. Sadly this twist was ignored, as Hiro ended up rescuing him.
The DCUElseworlds comic Kingdom Come shows a still-youthful Superman long after Lois Lane's death, and eventually builds up a romance between him and the explicitly-immortal Wonder Woman. He's still more or less in his prime, but his hair has begun to grey at the temples. And it's not like Lois died of old age, she was murdered by The Joker.
This is an issue in Watchmen, in which the immortal Dr. Manhattan is fully aware of the fact that his girlfriend will continue to grow old and he will not. His first girlfriend, Janey Slater, is also aware of this, and it eventually causes the end of their relationship when he leaves her for the much younger Laurie. Of course, Dr. Manhattan knowing beforehand that this would happen doesn't help things.
Lampshaded by Doctor Doom when he confronted Asgardian goddess Kelda who wanted to avenge the death of her beloved, Bill.
Doom:"He was mortal. You are not. Soon time would have struck him down as surerly as what you accuse Loki of."
ElfQuest just plain tears this up. The elves, magical immortal creatures, are the descendents of alien shapeshifters. After crash-landing on the planet with two moons, the aliens tried to survive: some by living as primitive humans, one by killing everything else, and one by turning into a wolf and becoming the ur-mother of a tribe of half-elf-half-wolf babies (a lot of which... didn't survive).
10,000 years later, the comic's main character Cutter, a direct descendant of the mortal wolf-blood line, pairs up with Leetah, a pure immortal elf. Drama ensues.
Heavily deconstructed during the second half of the main quest: another 10.000 years pass, during with most of the mortal elves decide to go into magical hibernation. One family of elves (who happen to look most like the "classical" elf of Western mythology) decide not to, and try to live a peaceful and fullfilling mortal life. The thing is — they just can't stop counting the years. Their youngest son, who was brought up by mortals in a tribe of immortals, decides that hibernation is a better alternative, because counting the years would make him go insane.
And then Winnowill, the series' main villain, discovers that her healing powers allow her to make mortal elves immortal. Against their will. And teaches Leetah, the hero's lifemate, that her own healing powers would allow her to do the same. After being kidnapped 10,000 years into the future, and believing (with good reason) that all of their friends are dead, Skywise (the hero's best friend) asks Leetah to make him immortal... only to find out several hours later that everyone they know is still alive.
At one point in the comic, Leetah is briefly — very briefly — tempted to "cleanse" Cutter in his sleep. She utterly hates herself for even thinking about it.
Aside from the main characters, there are many, many mortal/immortal pairings in ElfQuest, and even three cases of humans being adopted by elves.
Invincible's Dad, Omniman revealed that his kind can live for over a thousand years so he did his best to consider his human wife as more of a pet. X-Men villains Mystique and Destiny deconstructed this trope. Mystique doesn't naturally age but Destiny did, and since the two ladies met and fell in love in the early 20th century, Mystique had to helplessly watch her partner grow old, and she would have eventually died of old age had she not been murdered.
Though the original creators never alluded to it due to the crackishness of the pairing, shippers of Yuffie/Vincent from Final Fantasy VII have done this trope straight into the ground. Same with Vincent/Tifa...rare as it is.
In the Transformers Film Series fandom, it's been touched upon multiple times concerning the Sam/Bumblebee pairing. Rarer, but was still touched upon, in the SarixBumblebee pairing. Of course, after the revelation that she was a techno-organic, this pretty much vanished in fanfiction.
Indeed, this trope seems to be the general source of angst in Alice x Marisa pairings (the famous "Marisa Stole the Precious Thing" remix by IOSYS, who did the one above as well, contains the line "You and I are of a different kind"). Which is all good and well, until Fridge Logic sets in and you realize that, under current canon, Alice USED to be human but became youkai via magical research, and Marisa has access to Alice's notes/materials/Alice herself, AND Marisa has been independantly searching for an immortality potion.
The topic is likely to come up in any of the human/youkai pairings, as well as any immortal/youkai or immortal/human pairings, such as Keine x Mokou, since the immortals are completely immortal and can't even choose to follow their loves to the grave.
By far the most common is Sakuya, even without involving romance, as she is the only human in the relatively large Koumakan Cast Herd, meaning her expected lifespan is considerably less than their's. However, there is also speculation that Sakuya is a Lunarian, who are incredibly long-lived (Eirin and the Watatsuki sisters are at least several millenia old). Also occasionally inverted due to Sakuya's time-stopping powers.
A lot of the more...mature fanfic written for Star Trek: The Original Series touches on Sarek and Amanda's relationship and how Sarek would handle Amanda's death.
There are a couple of not exactly canon Star Trek novels that touch on them as well. The Vulcan Academy Murders is basically all about their relationship, and human/Vulcan relationships in general. It uses Applied Phlebotinum to age Amanda back down and give them another thirty years or so.
Other EU materials tend to follow this trope to its harsher conclusion probably because Star Trek The Next Generation confirmed that Sarek outlived Amanda. One Star Trek comic has Spock and Sarek converse about Amanda's increasing age and her growing health issues. Sarek states that while he was always aware of the fact that he would outlive Amanda, she has had such an impact upon his life that he finds the idea of spending the better part of a century alone to be unnerving.
Since Jareth and Sarah are the Fan Preferred Couple of Labyrinth, this is always an issue which is usually solved in one of several ways: most of the time, Sarah somehow becomes a fae (since everyone assumes Jareth is one), or sometimes just an immortal human. Some stories write it off by saying that all humans in the Underground age extremely slow. Less often, Jareth will actually become mortal and join Sarah aboveground, and very rarely, Sarah remains mortal and the relationship ends when she eventually... well, dies.
There is a lot of Sailor Moon fanfiction that deals with this concept, since only the manga says normal humans become long-lived in the future and even if they do live a long time, they still don't live as long as the senshi. One large series by author Bill K. even features two marriages within the senshi to normal men who, obviously, die way sooner than their senshi wives. Jupiter and Mercury react accordingly... and then hook up, so there's that.
Axis Powers Hetalia fics sometimes play with it too, usually focused on the relationship between national personifications and their most beloved historical/artistic/etc. figures. France/Saint Joan of Arc is probably the most common, given the later's five-second appearance in the show; but lots of famous rulers or national heroes get represented. (like Prussia and Frederick The Great, Austria and Wolfgang Amadeus Mozart, possibly Russia as the target of a Precocious Crush from Great Duchess Anastasia, etc.) There was even heartrending fanvid montage of a bunch of them floating around for a while.
Some Fairy Tail fics love to hook up Lucy with Loke, who is an immortal Celestial Spirit. Although most fics ignore the longterm implications, some like Lion's Pride focus on the trouble of love between a human and spirit.
The "Tragedy of Long Life" pool on Danbooru is dedicated to this trope.
This is a very common trope for shipping fanfics concerning the Princesses in My Little Pony: Friendship is Magic. Being the only two immortals (Alicorns) revealed so far, a common qualm about them getting into a relationship is usually along these lines, unless it is a Yuri pairing of either of them with Twilight Sparkle, who has enough magic to presumably make herself immortal if she wanted to, or with each other, a pairing known as Princest.
Due to it being unclear how long a dragon's lifespan is, there's speculation a Spike/Rarity relationship might wind up becoming this.
The Pony POV Series addresses this by having Present!Rarity admitting she knows of Spike's crush on her but is afraid of mentioning it for this reason, as its established Spike will outlive her under normal circumstances. Subverted in Dark World, where Rarity was made immortal by Discord, and thus a thousand years later, once Discord's control is broken, there's nothing stopping them from hooking up, which they eventually do. As an added bonus, Spike won't naturally outlive Rarity because of said immortality.
This story has Celestia revealing to Cadence that alicorns are immortal, and what that means for Cadence's upcoming marriage to Shining Armor, a mortal unicorn. Cadence doesn't take it well.
In Andromeda, two out of the three women on the main cast were an avatar of a star (it only became apparent rather late in the series, but she was believed to be older and immortal much earlier), and a just about immortal android. This made the trope common enough.
Generally (surprisingly) not brought up in Bleach fanfiction, mostly because it's usually Ichigo/Someone and he's a Shinigami as well and technically can "live" just as long. The age gap might be mentioned in passing, but that's it.
In Courage of the Spirit, which is probably the longest (and best) of what little Spirited Away fanfiction there is, Chihiro and Haku have this issue for most of the series. Naturally, it doesn't really get resolved completely in the end, although Chihiro does get more time than they thought.
The Harry Potter / Firefly crossover Browncoat Greeneyes has a 150 year old Harry paired with River Tam.
Films — Animated
Milo and Kida play with this in Atlantis: The Lost Empire. The former is twenty-something, while the latter is over 8000 years old. However, it's pretty strongly implied that the crystal at the heart of Atlantis is responsible for Atlanteans' longevity, so anyone who stays in Atlantis will enjoy it as well, regardless of their origin.
This is the reason given in Bluth's The Secret Of NIMH as to why Jonathan Brisby never told his wife anything about NIMH; the intelligence-boosters also slowed their aging. From a mouse's perspective, Jonathan was near-immortal, and he couldn't bear to tell her that he would watch her age and die while he stayed young. In the book it's specified that the anti-aging treatments were separate, and started after, the intelligence treatments. Nicodemus specifically discusses Johnathan's worries, confided to him, with Mrs. Brisby.
A creative (and soul-destroying) non-romantic variation on the theme appeared in Toy Story 2: Jessie, as an unchanging toy, "outlived" Emily's childhood. This is continued in Toy Story 3 where Andy has grown up and no longer needs his toys.
Films — Live-Action
The Hunger. Ancient, immortal vampire Miriam turns her mortal lovers into vampires, who then live with her for a couple of hundred years. They then wither and age rapidly to a near paralytic state, and she files them away in coffins in her attic. She does show some regret over this.
Reflected in the song "Who Wants to Live Forever?" by Queen which was, appropriately, written for the soundtrack of Highlander. A Mayfly December Romance is almost inevitable for the immortals in the film series, as they will naturally outlive any human love interests, and immortal-to-immortal romance is problematic too, as they are all engaged in a deadly battle royale with one another that won't end until only one is left.
Ray and Mary of Hancock also have to deal with the Mortal/Immortal issue. Although at first after the reveal it seems like it might be a dealbreaker for Ray and the issue is never explicitly resolved onscreen, they are seen at the end together and jokingly reviewing the truth about famous historical figures.
The Fifth Element, with Leelo being the December here. (Being at least a multiple of 4000 years old...)
Life in a Day, starring Michael A. Goorjian and Chandra West.
To extremes in The Man from Earth. Main character John Oldman appears to be 35, but is actually a Cro-Magnon caveman who has lived through 14 000 years. This naturally causes difficulty in his romantic relationships, as he has to abandon each woman after a few years so that people don't catch on to his immortality. References are made to various women he has loved, including one of the films main characters, Sandra, a historian in her 30's. Another one turns out to have been the mother of one of the other main characters. It is implied that John knew this all along.
The main character and narrator of The Green Mile turns out to suffer from this.
In We Are The Night, vampiress Charlotte acts pretty dissociated from her Sire Louise. Turns out the reason for that is that Louise sired Charlotte despite the latter being happily married to a man she loved and having a child with him. Later in the movie we see Charlotte visiting her daughter in the hospital. While Charlotte is still youthful, her daughter dies of age right in front of her eyes. Also somewhat counts for Louise, since the vampiress who sired her died and left her restless and heartbroken.
In the second Night at the Museum movie Genki Girl, Manic Pixie Dream Girl Amelia Earhart, or rather a magical construct of the XX century aviatrix, tries actively to pursue a romance with Larry Dailey, the protagonist. Larry instead tries actively to spurn her (with little success...), knowing that the magic infusing her with life will wane at dawn, thus wanting to avert this scenario, sparing Amelia the knowledge of her overtly short "new lease" at life.
In Fred Claus, Santa Claus's immortal immediate family suffers from what the article calls the "Highlander Complex": "They will have to watch their friends and relatives wither and die right before their callous, eternal eyes."
In the movie adaptation of Tuck Everlasting, this is the reason for Miles Tuck's bitterness; his wife, daughter, and son all died while he stayed immortal.
"I couldn't die. Like Little Anna. The influenza took her before she was fifteen. And Bo. He'd be almost eighty now if he were still alive. And my sweet... my sweet young bride. She died in an insame asylum. Old and alone. But I'm still here... I'm still here."
These happen several times in J. R. R. Tolkien's Middle-earth legendarium, in the form of rare but significant elf-human pairings. Elf-human mixed couples overall are very rare (known are five and a half, and only three got together permanently). The main angst isn't just because elves live forever and humans don't — the two races do not share the same afterlife. Elves, even if killed, travel to the Undying Lands in the west and are usually granted resurrection, eventually. Humans, upon death, disappear from the world and their fate is unknown (presumably they go to the Christian Heaven). And elves by nature mate for life, so they usually can't get over it and marry one of their own kind afterwards.
In some cases the angelic Powers give the immortal one of a pair the choice to become mortal, which means they can spend eternity separated forever from their entire family, rather than from their spouse. And the children of most elf-human pairings were historically given the same choice, which potentially separated them from their families, also. Like all resolutions in Tolkien's works, either choice is bittersweet at best.
One of the better-known cases is the romance between the mortal Aragorn and the immortal maiden Arwen Undómiel, daughter of Elrond, in The Lord of the Rings. It was a relatively minor subplot in the novel (their full story is in the LotR's Appendices), but became a much larger part of the later films. During the book Aragorn is 88 (young for a human of Númenórean ancestry), while she is 2798 years old. And when they first fell in love, he was a teenager and she was still over 2700 years old.
To be specific, Arwen is descended from a line of mixed heritage involving humans, elves, and even a Maia (angel). Her paternal grandparents, themselves half-human, chose to be immortal, and the opportunity to choose was passed on to their descendants. Aragorn himself is distantly descended from the same ancestors. The first King of Númenor was Elros... Elrond's brother! There were dozens and dozens of generations in between, which is itself another example of this very trope. Apparently if the parents decide to be immortal, their children can still pick, but if they decide to be human, they can't... this has some connection to Tolkien's Christian concept of death as the "Gift of Men," since presumably it involves actually going to Heaven. Aragorn, having Númenórean ancestry, lives roughly 210 years. But Arwen may have outlived him by months or years — at the end he was aging and decided when to die of free will (an ability only Númenóreans get), while Arwen was still young and not ready to go. Arwen left her daughters and son behind to go to Lothlórien and spent her remaining time alone.
There are two main elf-human pairings: Beren (human) and Lúthien (half-elf, half-angel); and Tuor (human) and Idril (elf). Lúthien chose to become mortal; Tuor supposedly became immortal. These four were in turn the ancestors of Elrond and Elros, and thus of Arwen and Aragorn. Beren and Lúthien's son Dior may have had a Mayfly December Romance with his elven wife Nimloth — he was killed young, but he grew up at the rate mortals do and it isn't clear how long he was going to live, or if he would have been given a choice to be immortal. Their fates after death are unknown.
There is one male elf/female human Star Crossed couple — in fact the very first elf-human romance known to us: Aegnor (Galadriel's brother) and Andreth (a woman of the Edain). Aegnor pretended not to return her love, so he wouldn't have to watch her age, though it probably broke his heart to do so. He also feared that if the Siege of Angband broke then he would very likely die in battle (and he was right — she outlived him).
Another human-elf pairing that didn't work out was that between Imrazôr, a Númenórean who settled at Dol Amroth, and Mithrellas, a Wood Elf who got totally lost in the mountains when her people fled from the Necromancer in south Mirkwood. He and his folk sheltered her and they got married, but after having two half-elven kids she just... up and left him. Elves aren't inclined to marry people they don't love, and there's no suggestion that he married her against her will, so maybe she just didn't want to watch Imrazôr and the children die of old age.
In The Unfinished Tales, this trope played beautifully in the marriage of Aldarion and Erendis on Númenor, lost home of the Dúnedain. Númenóreans had a typical lifespan of 100 to 200 years, but those of the royal house lived two to three times as long. Aldarion was the 6th king of Númenor; Erendis his wife was not from the royal house. Aldarion's long lifespan and his tendency to make multi-year voyages while Erendis continued to age (relatively) rapidly drove a massive, nasty wedge in their relationship. Their problems in turn embittered their daughter, the future queen Tar-Ancalimë. This proved to be the first small step in the eventual downfall of Númenor.
The Lord of the Rings: Faramir and Éowyn probably experienced this trope, since he had some Númenórean blood (and some elven blood from Mithrellas), and despite being 13 years older than her, he could have outlived her by several decades. We aren't given a date of death for Éowyn, but Faramir lived well into his second century and it's extremely unlikely Éowyn lived nearly that long. Note that all of the mixed couples in Middle-earth human/elf, despite the fact that both hobbits and dwarves are usually on good terms with their immediate human neighbors and with one another. Dwarves are an unrelated species and can't breed with non-dwarves, but hobbits are just small humans. And in the First and Second Ages when they were on good terms with other humans, the Drúedain apparently didn't marry outside their own people either.
In Tuck Everlasting, not-actually-17-year-old Jesse Tuck tells Winny to drink magic water to gain Immortality when she reaches that age. She gives the water to a toad, ages normally, lives a happy life and dies. Jesse's older brother Miles was once married. When his wife realized that she aged while he stayed twenty-two, she accused him of selling his soul to the Devil.
While it's not really romantic (unless you get into fanfiction), dragons in the Temeraire series are capable of far outliving their much-adored riders. It's even suggested to Laurence that he hurry up and have a child, so that the transition will be easier when Temeraire is forced to move on to another captain after Laurence's death.
Philip Pullman's His Dark Materials features the witches, who live for roughly a thousand years, and so are continually having their lovers, husbands, and mortal sons dying on them (their daughters are always witches). One of them describes this as being very painful, and suggests that they eventually die when they can't take losing anyone else.
Drizzt Do'Urden, in his series of novels by R.A. Salvatore, spends much of the series trying to prevent himself from getting close to his human friend, Cattie-brie, knowing that he will outlive her by centuries, but eventually falls for her anyway.
The premise of the fantasy novel Sirena, where the protagonist is one of the Greek Sirens who will become immortal after having sex with a human man. Falling in love with said man, however, makes the irrevocable gift a curse.
Seen in the Inheritance Cycle, where age is given as the main reason Eragon can't have a romance with Arya. This doesn't really make sense, though; Arya may be eighty or ninety years older than Eragon, but as a Dragon Rider he will have a similarly extended lifespan.
It's because that while yes, he may have a lifespan as long as an elf's, as is pointed out to him.... he hasn't yet. From her perspective(and Elven law) he is still technically a child and being with him (for her) would pretty much constitute statutory rape(not that that is a thing in Paloniverse, but same idea), because elves do not become fully adult until 20, and even if she waited until then it would be like someone in their mid twenties dating a "barely legal" teen(which happens admittedly, but generally causes some raised eyebrows and probably some questions at least). On top of that, while she is young by elven standards she is still far more mature than he is and knows it. In the end of the last book the plot point is resolved by them finally getting together. Then Eragon leaving.
Ended rather badly in David Eddings' Belgariad. Polgara fell in love with a Wacite Arend, her Champion, when she was nine-hundred-odd years old and he was... oh, about thirty. When the Asturians attacked Vo Wacune, her father hustled her out of the city, leaving Ontrose to die in the fighting. She hasn't forgiven Belgarath yet, even though he did raise a valid point about him dying a couple millennia before she would anyway, and she carries a torch for him still, despite her marriage to Durnik.
This trope is touched upon in the Robert A. Heinlein novel Time Enough For Love, which has as a main character an essentially immortal man, Lazarus Long (born Woodrow Wilson Smith, around the turn of the 20th century). Specifically, the story involving Dora. She's fully aware of the trope in play, and insists on playing it straight. Lazarus offers to move with her to a planet with rejuvenation facilities and extend her life as long as possible. She refuses, saying she wants to spend the life she was naturally given on her home world with the man she loves. Lazarus specifically states that it's a bad idea for long-lived Howards such as himself to marry people with normal lifespans (or "ephemerals"). Dora was possibly the only time he broke that rule.
This subject causes much angsting in the Dragonlance novels, in which Tanis Half-Elven loves both an elf who would long outlive him and a human who would die while he was still young. In case people want to know, he chose the elf, who outlives him due to a more mundane reason.
The Xanth novels are full of this : X(A/N)th (male demon)/Chlorine, Demetria (female demon)/Veleno, Jewel (nymph) / Crombie, and others. Averted, as Xanth is also absolutely full of Fountains of Youth. And X(A/N)th, being literally omnipotent can do anything he damn well pleases, including extending a human's life indefinitely.
One of Clive Barker's Abarat books has an unusual twist on this idea: on the island of Nonce, where the heat and storms cause the rainforest there to grow and decay incredibly quickly, a man has fallen in love with a plant. Plant-woman. However, her life is very brief compared to his (a few hours, at most, with the island's weather), but she also always produces a seed that is a fresh new version of herself. So he spends his whole life clambering over the island all day, looking for her newest sprouting, even though she won't remember him, so that for however brief, they can be together.
Bella of Stephenie Meyer's Twilight series is very conscious of this trope and has spent most of three books trying to convince her immortal vampire boyfriend to bite her to avoid this. In the fourth book Bella does end up turned into a vampire after her pregnancy makes her very sick and Edward performs an Emergency Transformation or she'll die.
In the Star Trek: New FrontierExpanded Universe series, Selar (a Vulcan with a lifespan of about 250 years) mates with a Hermat (with a 40-year lifespan...and both sets of genitals). Selar carries a child to term...and it turns out he has the lifespan of an Ocampa (10 years, if he's lucky.) In the most recent novel of the series, Treason, this gets fixed, though ironically (for multiple reasons) Selar dies before this happens. It's not a happy book.
A Star Trek: The Next Generation novel, Immortal Coil dealt with this in connection with Data. Taking place not long after First Contact, the plotline includes Data's emotional realization of what he's always known intellectually — he will almost certainly outlive all of his friends on the Enterprise. And then another set, and another. The actual plot of the book assuaged these fears by introducing a league of artificially-created organisms, which Data could retire to whenever he wants. And then Nemesis happened...
In the Star Trek: The Lost Era novel The Buried Age, Jean-Luc Picard enters into a relationship with an alien known as "Ariel" (real name Giriaenn). Ariel is effectively immortal, and has been alive longer than humans have existed as a species. Her relationship with Picard was genuine while she had amnesia; when her memory returned, though, she began stringing the "innocent child" along as part of her master plan. Being as old as she is, she's an complete expert at manipulation, plus her people are naturally designed for it anyway.
The long-lived Tiste Andii, Korlat, and the already fairly old Human, Whiskeyjack, in Malazan Book of the Fallen. In fact, it's stated that Tiste Andii tend to have relationships with shorter-lived races, finding 200 years of marriage to their own kind wearisome and a bit pointless.
Stardust features this. Mortal human man, immortal ex-star in human woman form -> dead man and still immortal star regent of their kingdom. In the movie they got around this handily by playing on the "heart of a star" thing, i.e. one who has the heart of a star will live forever. In the films they claim that Yvvaine had "given" her heart to Tristan/Tristran, and therefor they will both live happily forever after.
The Hero And The Crown has an interesting variant. Luthe is a super master mage and at least 100 generations old, probably older. Aerin is 20 years old but magically powerful, and capable of being "not quite mortal" herself. They have a May-December Romance. But she's also in love with the 30 year old man who helped raise her, and he's basically mortal. So Aerin goes off to marry Tor (and co-rule her country with him) until he dies, and then go back to Luthe who'll still be around and probably still in love with her.
Anne Bishop's Black Jewels Trilogy has the pairing of Daemon and Jaenelle. Daemon was about 1700 years old when Jaenelle was born. This is a source of much frustration when Daemon first meets her and she's only twelve. Balanced a bit by Jaenelle being The Chosen One and therefore closely tied into the world's magic and wisdom and Daemon's race being long lived so that he's the equivalent of about 30 years old. But they don't actually hook up till she's in her 20's since Daemon is most definitely not a Lolicon. That and he's wandering around out of his mind because he thinks he killed her. Also Jaenelle is 100% human, so she'll live maybe sixty/seventy more years after The Queen of Darkness and Daemon could conceivably live to be twice or more his original stated age of 1700 years. There's also Daemon's Mixed Ancestry half-brother Lucivar who is about Daemon's age and marries a woman who appears to be only about 300 years old.
Subverted in The Redemption Of Althalus, where the goddess Dweia and human protagonist Althalus fall in love, but the problems are resolved early on when Dweia reveals her ability to control time, letting her and Althalus to stay together as long as time itself exists.
In Susanna Clarke's Tom Brightwind or How the Fairy Bridge Was Built at Thoresby (set in the Jonathan Strange & Mr Norrell universe), the eponymous character has one of these with his human wife, but it sours due to his fairy fickleness: "At fifteen... Tom had constantly compared her to a kitten. In her twenties she had been a swan; in her thirties a vixen; and then in rapid succession a bitch, a viper, a cockatrice, and finally, a pig. What animals he might have compared her to now no one knew. She was well past ninety now and for forty years or more she had been confined to a set of apartments... while her husband waited impatiently for someone to come and tell him she was dead."
Norman Spinrad's short story Deathwatch examines a future in which humanity develops a mutation that prevents aging, but not everyone has it. Warning: this story has been known to cause blurry vision in even the most stoic readers.
In the Mercy Thompson universe, werewolves are immune to old age and disease and can live for centuries or millennia. Bran and Samuel are at least 1300 years old... barring injury and insanity. The werewolf Samuel Cornick has had three human wives, each of whom died of old age. Another, unnamed werewolf in Cry Wolf is shown kissing his elderly wife who was initially mistaken for his grandmother. In Hunting Grounds, Arthur chooses to have his wife assassinated by vampires rather than suffer seeing her die of old age.
Several times in the Symphony Of Ages. The Cymrians live shorter lives with each generation, starting with immortality at generation one. This presumably causes true mayfly romances, but the most visible case is Tristan Steward and his consort Prudence. While they grew up at about the same rate, she starts showing her age and he remains youthful. A much more complicated version arises among Rhapsody, Ashe, and Achmed. Rhapsody, a first-generation Cymrian and lirin, will live forever. So will Achmed. Ashe, however, is part dragon and a third generation Cymrian, granting him a lifetime that will probably last a few thousand years more before he either dies or reincarnates as a full dragon (somehow presumed to end their romance, although other dragons have loved humans before). Achmed loves Rhapsody, or at least believes she is the only possible suitable mate for him, but is content to wait millennia for Ashe's eventual removal. (Somewhat surprising, given that he is a cold-hearted assassin and probably quite capable of killing Ashe if he felt his need for Rhapsody required it.)
Subverted and reversed in Glen Cook's Dread Empire series. Varthlokkur is an immortal wizard who is able to use his magic to find the woman he will love. He waits millennia for her to be born, getting married once or twice for fun along the way. His son from one of these marriages ends up winning her heart first. It's okay, though; Varthlokkur can ensure that she lives forever and his son doesn't.
In Conn Iggulden's Emperor series, Caesar has a love affair with Servilia, his best friend's mother. Problematic since he needs a son and she is too old to get pregnant.
Salamander/Ebañy is a half-elf whose wife died and is reincarnated in a new body. (Reincarnation is a big part of the Deverry books.) When he finds her again, she is a teenager, and he's about 100. He sees no particular reason not to marry her again — as he points out to his doubting friends, there aren't that many half-elfs around, and this way they might actually live to grow old together. They don't.
Dallandra and Aderyn, she is an elf, he is human. Made worse by her spending time in the Land of the Guardians, where time runs slower, then a partial subversion when Evandar gives Aderyn an elven life span. Unfortunately he forgets to give him elven youth to go with it.
Completely averted with the human Princess Carra and the elven Prince Dar. Evander remembers elven life and youth this time.
In Dan Simmon's Hyperion, Merin and Siri from the sub-story, "The Consul's Tale", are engaged in a Mayfly December Romance where Merin, who is a crew member on an interstellar construction ship responsible for building a farcaster gate above Siri's homeworld of Maui-Covenant, must watch his love progressively wilt away as he travels back and forth to the planet at relativistic speeds, all the while remaining young himself.
In the Night Huntress series, this occurs between Bones, the two-hundred-twenty-year-old vampire, and Cat, the twenty-year-old half-vampire. Later Bones reveals that between her half-vampire blood and the occasional drinking of vampire blood, Cat can live for as long as he can, making this a May-December Romance.
In The Apocalypse Troll by David Weber, Ludmilla Leonovna is effectively immortal thanks to the Healing Factor her symbiote grants her. So when she falls in love with a normal man, she's very aware of the fact that she'll outlive him. Then when he's mortally injured in a fight with the Big Bad, the only option is a risky Emergency Transformation via blood transfusion, injecting some of her symbiote-laden blood into him. This kills 99.99% of the people it's done to, so she wouldn't try it except for the fact that he'll be dead in five minutes if she does nothing.
In other series by David Weber, we see this pop up alot in platonic forms. In Safehold Nimune/Merlin is in a robot body with potential life in the thousands of years, she/he's injected his/her friends with nanites that will give them perfect immunity to infectious illnesses and cancer, but doesn't dare give them life extension treatments because their very public life makes it impossible to do so without it being noticed. In The War Gods we have Wencit whose well over a thousand years old. He also knows the results of how all crossbreeding between the 5 races works, and the other races all out live men. Half-elves only marry half elves or full elves else their children will be too human and not get the Elves life spans.
Also comes into play, though not in a romantic sense, in the Honor Harrington series.
Honor herself, and everyone else in Manticore, has ready access to Prolong, a treatment that pushes human longevity up to several centuries...if it's administered at a young enough age. The people of Grayson, the planet which adopts Honor as its own, only get it in the present, and many of the people she gets close to are too old for the procedure and thus she'll outlive them by a considerable margin.
One particularly tragic case did involve romance: Bernadus van Dort and Suzanne Bannister. He was a prolong recipient, and she turned out to be too old for the treatment. What made it worse was that she didn't die at a ripe old age, with her husband still looking young. Her transport ship vanished, probably thanks to a pirate attack. Oh, and their daughters were aboard as well. The whole thing also created a rift between van Dort and Suzanne's brother Trevor, and Trevor's friend Steve Westman, which spills over into the plot of Shadow of Saganami.
One of the eponymous AI tanks in the Bolouniverse bitterly reflects upon this when she realises that her commander has been treating her as a human woman, not a multi-thousand-ton, fusion-powered, functionally immortal battle tank, and that she loves him back. She gives no hint of this angst to him and keeps it strictly platonic.
In Octavia Butler's Wild Seed, protagonist Anywanyu had many husbands throughout her unnaturally long life, both she and her lovers seemed at terms with this.
In Myst: The Book of Ti'ana, Atrus (not the one from the game, but his grandfather) starts to become romantically involved with Anna. His father Kahlis reminds him that the extended D'ni lifespan will mean that he will outlive her by centuries. However, Anna eventually outlives him due to a Heroic Sacrifice.
Played straight and subverted in Cordwainer Smith's The Queen of the Afternoon. Laird, who is basically immortal due to rejuvenation treatments, marries a woman who is unable to be rejuvenated. Whe she grows old and dies, he married her sister, who has the same problem, but when she is dying, he realizes that he can't bear going through the loss again, so he refuses the treatment, ages hundreds of years in an hour, and the two die together..
Black Dagger Brotherhood: Both Mary and Jane were humans hooking up with vampires, so Mary became immortal to make up for being barren and Jane became a ghost.
In Cassandra Clare's Mortal Instruments series, Alec Lightwood is eighteen, while Magnus Bane is 800 years old. The fact the Magnus is an immortal warlock while Alec is human becomes a plot point in the fourth book of the series.
The Adoration of Jenna Fox has Jenna, who is immortal because she is an Artificial Human, marrying Ethan and having his child before he dies. Jenna says she'll end her life when their daughter reaches a certain age, so she doesn't have to watch her die.
In The Wheel of Time, this trope is why Aes Sedai rarely marry: they know that they will far outlive their husbands, and probably their children.
And grandchildren. And great-grandchildren too. The will most likely die around the same time as their great-great-granchildren. The average age for a channeler who has sworn on the oath rod is somewhere between 2 and 3 centuries. For one who hasn't its somewhere around 6.
In John Green's new book The Fault In Our Stars, Hazel, the sixteen-year-old protagonist, diagnosed with terminal cancer, has an uncertain number of years to left to live, but probably not many. Then she meets and falls in love with a seventeen-year-old Augustus, who is well into remission and will likely live a normal number of years. However, Augustus' cancer recurrs, every reader's heart breaks, and Hazel is the one left behind by a lover's death.
In Holly Black's Modern Tales of Faerie there is unbelievable quantities of this, most notably in Valiant between Val and Ravus, who actually comes out and says that she will be dead in "one faerie sigh", but they are still hooked up.
Nicholas Sparks' A Walk to Remember features a young man who marries his high school sweetheart when she's dying of cancer, leaving her dead at the age of nineteen mere months after their wedding while he lives a normal lifespan.
The Kantrishakrim of Tales of Kolmar live for two thousand years, optimally, and mate for life. When one falls in love with a human this trope is brought up; in fifty years the human will be in her dotage or dead, and plenty of Kantri spend that long meditating or in contemplation; fifty years is also about how long it takes for a Kantri to heal from a major wound, which they do while in a comatose state. Oddly, the fact that they have ready access to a kind of elixir of youth is never mentioned by either party. When the Kantri seems to be dying, the human thinks that he should have mourned over her grave for a thousand years. Instead, though, he becomes human, though he still has Kantri blood; later on her blood is changed to be like his, so if that conveys longevity neither is too likely to outlive the other.
In the Northland Series, Laika ends up marrying Koschei the Deathless. They both realize that she will not live forever, but they simply decide to accept that. They also are implied to have children, so Koschei will at least not be lonely in the future.
Averted in the John Carter of Mars books — you'd think this would apply with the protagonist falling in love with a Martian with a life span of hundreds of years, perhaps potentially forever, but Carter also becomes immortal for the same mysterious non-reason that he gets transported to Mars.
In The Host, Wanderer is an alien soul who is thousands of years old. She proceeds to fall in love with a human man. However, since she is in a human body, she can age and die naturally alongside him.
Comes up frequently in Tom Holt's works, what with all the gods, immortal spirits, shapeshifting dragons, and whatnots, who often end up romantically entwined with mortals, willingly or otherwise. Some specific examples:
In Expecting Someone Taller, most of Malcolm's romantic entanglements after receiving the Ring fall into this category, especially his eventual engagement to the Valkyrie Ortlinde.
Nothing But Blue Skies has an ancient Eastern Dragon take human shape because she's fallen in love with a human—although he's completely unaware, and she's too shy to tell him.
In Djinn Rummy, an immortal genie falls in love with his human female master, although it's because of a love potion.
Gladia and Elijah from Isaac Asimov's Robot Series. He is an ordinary human, but she is a Spacer - a human descended subspecies living up to four centuries. They first meet in The Naked Sun, with some Unresolved Sexual Tension between them toward the end. In The Robots Of Dawn, they meet again. This time, the relationship and love between them is definite, but there is more than just this trope separating them (the relationship between the two races is rather strained, not to mentioned Elijah is married). Their last meeting is about five years later and only lasts a few hours - with Gladia married to another man by then as well, and the marriage turned out to last a century and a half. She wants to visit him one last time before he dies, but he sends a message begging her not to, not wanting her to remember him as an old man. By the end of Robots And Empire, Gladia is in a relationship with Elijah's great-great-great-great-great-grandson, and both are fully aware she might outlive him by 50-100 years as well.
The Last Unicorn has two examples of this: one serious, one not. When the tree falls in love with Schmendrick, it's treated as a joke. Several chapters later, Amalthea and Lir fall in love. It's explicitly stated that Amalthea still has feelings for him after she's turned back into a unicorn.
In Marion Zimmer Bradley's Darkover series, this is a significant part of why humans and the chieri do not have long-term relationships, even though they can and do interbreed.
Live Action TV
The Doctor in Doctor Who tends to avoid becoming too close to his companions, due to what he calls "the curse of the Time Lords" — a companion could spend their whole life with him, but he couldn't spend his whole life with them (see quote), being "immortal, barring accidents".
An episode of the new series, "School Reunion", explored this issue in detail by focusing on aging ex-companion Sarah Jane Smith, while another, "The Girl in the Fireplace", gave the Doctor a poignant Mayfly December Romance with Madame de Pompadour.
The Doctor: I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone that you .. Rose: What, Doctor? The Doctor: You can spend the rest of your life with me... but I can't spend the rest of mine with you. I have to live on. Alone. That's the curse of the Time Lords.
The Third Doctor's relationship with Jo Grant had shades of this. Their relationship was never quite romantic (unless you consider being tied up together romantic), but he did seem awfully disappointed when she ran off and married a man she described as a younger version of him.
Worse still, according to Jo, she never stopped running, hoping she'd see the Doctor again someday.
Subverted in "Flesh and Stone", when the Doctor raises this trope as a defence to Amy's romantic overtures: "Aw, you are sweet, Doctor, but I really wasn't suggesting anything quite so long term."
Played straight in the Doctor's relationship with Rose Tyler, which did become romantic and had a bittersweet ending.
Played with with the Doctor's latest companion, Clara, who teases the Doctor about the TARDIS being a "snogging booth."
Played to the very end with the Minbari Delenn and the Human Captain Sheridan. Not only do Minbari outlive Humans by decades, but Sheridan's return from being Not Quite Dead shaved thirty-something years from his lifespan, leaving them a scant 20 years of married life. Notable for also showing Delenn decades after John's death.note A bit of irony is that it may in fact have been Sheridan that out "lived" Delenn as his body was never found, only a sealed empty ship that had him entering it but never exiting it. But then, Valen's body was never found either, even though he was entirely mortal. The batting record for members of the One being ascended to a higher plane are therefore 2 out of 3. Draw your own conclusions about Delenn's ultimate disposition.
Lorien mentions this trope in a conversation with Susan Ivanova. He is the First One, and the Last of His Kind. He says that being immortal is to leave behind everything you care for, and that love is transitory. According to him, only short-lived species can imagine that love is eternal. He recomends that she embrace that wonderful illusion.
Due to the great difference between Vulcan and human lifespans, Spock's father was sure to outlive his mother by many years. (This wasn't addressed much in The Original Series.) Sarek has a different human wife, Perrin, in The Next Generation, so Amanda must have died between the generations, but we don't know when (Though the Extended Universe novel Sarek includes her death from natural causes shortly after the events of the sixth movie).
The same issue comes up with Klingon/human relationships, as the human lifespan is longer than that of Klingons. Again, we've never seen this addressed, but that's probably because humans who marry Klingons (especially Worf) don't usually get to die of old age.
Worf's more prominent (and only non-Klingon...er...Half-Klingon) marriage was to a joined Trill, which confuses, and, depending on one's interpretation of the joined Trill/Symbiont relationship, laughs in the face of the whole matter. (None of the major Daxes have had any problem with long-term couplings with persons who aren't joined Trill.)
On Star Trek: Deep Space Nine, the changeling Laas brings up this trope while trying to convince Odo (a changeling whose species has an incredibly long lifespan) to leave Kira (a Bajoran with a roughly human-like lifespan) in order to join Laas in locating the rest of The Hundred.
Laas: It won't last.
Odo: The fact that your relationship failed doesn't mean mine will.
Laas: True. If you're very lucky, you'll get to watch her grow old and die.
For most of the first three seasons, Kes, whose race has a nine-year lifespan, was in a relationship with Neelix, whose life expectancy was apparently like humans. Kes's replacement, Seven of Nine, had her own age issue — she was only six when the Borg assimilated her, making relationships with the adult crew somewhat dubious. The exception was the Doctor, who was technically only a few years old himself. Thus Doc/Seven (teased, but never followed through on — a true shame as it was probably the only 'ship in the show that demonstrated any real chemistry) was a rare example of a Mayfly December Romance that looked ickier than it was.
Explored also in the episode where Kes is traveling backwards in time, so we see an alternate future where Kes is married to Tom Paris while his friend Harry Kim is married to Kes's daughter.
The Doctor gets another in the final episode that has scenes set in the future, he married a human woman, butthen that future was changed when Janeway went into the past to get the ship home sooner
An early first season episode of Bewitched focused specifically on the issue, with Samantha (apparently) admitting she will still appear to be in her twenties when Darrin is in his seventies. The dilemma is solved when Samantha uses her magic to age along with Darrin. It is unclear whether this would include her actually dying with Darrin instead of living centuries after his death, but for her to shorten her lifespan so radically to age as Darrin ages would provide yet one more reason for Endora's continual efforts to turn Samantha against Darrin.
Buffy's relationship with Angel was a mortal/immortal case. This didn't bother them much, until the Big Bad and then Buffy's mom brought it up just in time for the season finale. Angel ended up leaving Buffy for her own good... and the age issue in human/vampire relationships was apparently forgotten by all concerned at that point. (It certainly didn't figure in either Buffy/Spike or Angel/Cordelia.) Of course, the only reason they came up for that reason of Angel leaving Buffy was because they had to somehow get him out of Sunnydale for his spinoff show.
It's fairly clear Buffy never intended anything other than a sexual relationship with Spike, and highly doubtful Spike would have thought of the issue at all. And Angel, by the time he started getting tentatively involved with Cordelia, was planning to become human again anyway.
The initial age problem was enhanced by Buffy being a high school teen; Buffy/Spike and Angel/Cordelia had slightly more adult girls. Buffy provides another variant: given probable Slayer life expectancy (a few years, cut short by violence) even an ordinary human romance could be Mayfly December, at least on average. Also, given the fact that Angel was fighting side-by-side with Buffy, his survival chances weren't much higher. It would have been entirely plausible for him to die in battle, leaving Buffy alive.
It should also be noted that by the time Angel starts getting involved with Cordelia she's part demon, and a damn powerful one at that. So, it's a high probability that she was as immortal as he was at that point. It should be noted that the life expectancy of each character, in terms of their relationship, was rarely brought up. Angel's stated reason for leaving Buffy was that his curse (making him unable to ever be truly happy with her, which obviously would put a strain on both of them) and his nature as a vampire (sterile and lethal reaction to sunlight basically rules out any kind of normal family situation) made him unable to give her the life she deserved.
Played with for Anya/Xander. They never had a Mayfly December Romance relationship, but only because Anya lost her immortality for the duration of their relationship, and got it back after they broke up. Notable in that she lost her immortality in an unrelated fight with Giles, and got it back only because she was so furious about the break up.
Could have happened in Charmed between Piper and Leo, given that witches have normal human lifespans and whitelighters are immortal, if Leo didn't choose to become human.
Stefan and Elena from The Vampire Diaries are another example of an immortal/mortal case. Stefan is a 164 year old vampire and Elena is a 17 year old human girl, who will grow old and eventually die. Stefan is immortal, will never age, cannot grow old and is immune to all human illnesses. Therefore, Stefan's survival rates are significantly much higher then Elena's, who is a vulnerable human. Elena is susceptible to human weaknesses such as illness, aging and possible death. However, despite the difference in species and age difference, Stefan and Elena don't seem to mind. Their love for each other seems to transcend all barriers. Although, due to the differences in age and species, there has been some tension between Stefan and Elena. Stefan and Elena have talked about their age difference and also, the possibility of Elena choosing immortality in order to spend an eternity with Stefan. However, Elena has told Stefan that even though she loves him, she doesn't and never wants to be a vampire and would rather stay human, grow old, have children and live a normal human life. However, the age difference and the concerns over Elena's safety and vulnerabilities as a human don't seem to be a problem any longer now that Elena has recently become a vampire.
Their potential difference in lifespan has been a source of angst to Superman and Lois Lane in various incarnations. In particular, an episode of Lois and Clark focused on it, with Clark admitting he had no idea how fast he would age — if he aged at all. It was possibly resolved when Superman gave up some of his youth to rescue Jimmy from Rapid Aging and also to de-age the villain of the day into a baby, and one of the characters suggested that Superman had given up enough of his youth for it to no longer be an issue. Of course, since the show didn't run that long, we have no idea if Lois still has to worry about this...
Partly averted in New Amsterdam, where the premise is that the main character will be immortal until he finds and weds his true love. Only partly averted, because any relationship with someone other than his true love would fall into this trope, including platonic relationships such as his 67 children. He has mentioned being careful to avoid siring more children specifically because of not wanting to watch them grow old and die before him.
All of Jack's relationships in Torchwood. Unless we're counting The Doctor, of course. But whom are we kidding? He's never around.
According to a photograph seen briefly at the end of "Something Borrowed" Jack has had at least one spouse he has outlived.
In "Small Worlds", we see Estelle, an old woman who tells Gwen about how she was in love with Jack's father decades before; turns out it was Jack, and the affection is still obviously there on his end despite his being, physically, several decades younger. His reaction to her death at the end of the episode was heartbreaking.
Children of Earth: "Day One" introduces us to Alice Jack's daughter who looks about the same age as him as she hasn't inherited his immortality.
It's been addressed now, in the radio play The Dead Line.
Ianto: But let's be honest, Jack. I'm nothing more than a blip in time for you, Jack. Everyday, I grow a little older. But you're immortal. You've already lived a thousand lifetimes. How could you watch me grow old and die? How can I watch you live and never age a day?
Also addressed in a conversation in Children of Earth, Day Three. And then, of course, Ianto dies.
In the last two episodes of the first series of Being Human, vampire Mitchell runs into an old flame of his, who is now a fairly old woman. He gives her the option of becoming a vampire, which she turns down, putting him on the path to get out of his Heroic BSOD and almost Face Heel Turn.
On Heroes, Sylar seems to believe that this inevitability is grounds enough upon which he can begin to build a relationship of some sorts with Claire. Despite the fact that, at present, she's not even eighteen where he's roughly thirty—and that he's a sociopathic murderer and she hates him with a fiery passion. He promises she'll get bored of trying to kill him one day...
As with the film series, all Immortal/mortal relationships in the Highlander TV series are this. It's spotlighted in the season one episode "Studies in Light", in which Duncan encounters a mortal woman who was his lover fifty years ago.
On The Almighty Johnsons the entire Johnsons family are reincarnations of Norse gods but only have limited godly powers and thus live a normal human life span. The one exception is Olaf who is the god of rebirth and thus will stay young forever and cannot die of natural causes. The current generation of Johnsons are actually his grandchildren and he pretends that he is their wayward cousin. By his count he fathered 20 children so far and had to abandon them and their mothers when it became too apparent that he did not age. These days he no longer tries to have a longer lasting romantic relationship.
The film "May Flowers" made with The Movies features a romance between a normal human man and a woman (the title character) who is an ancient species, timeless but not ageless. Over the course of a year she ages an entire lifetime and dies at midnight every New Year's Eve only to be reborn as a baby seconds later. May finds husbands to take care of her during this time but does not stay married to them for more than 50 years.
This was the theme of the music video for Paula Cole's big hit "I Don't Want to Wait," an immortal woman who had lovers in several time periods who each died.
The song "Puff The Magic Dragon" describes a Mayfly December friendship between the dragon and his best friend in its final verse. It breaks the dragon's heart (as well as its listeners'). As with the Toy Story examples above, the issue is that he and all his adventures 'make way for other toys'.
Sonata Arctica's song 'Under Your Tree' describes this kind of frendship,originaly it was written because Tony Kakko's (main vocalist) dog,who he raised since he was a pup,died,and the day he was born he planted a tree.Now many years later the only thing was left is the tree and the memories.
Josh Ritter's song "The Curse" depicts a relationship between a mummy who is blessed/cursed with immortality and the archaeologist who enters his tomb and "awakens" him. She dies toward the end of the song. The music video is one hell of a Tear Jerker.
Half-subverted in Dungeons & Dragons. While half-elves tend to have somewhat downbeat childhoods, their parents can apparently have quite happy lives together. The human is attracted to the elf's grace, the elf to the human's energy. It's implied that the human gets a partner who doesn't age for the duration of their relationship, and the elf gets a satisfying if short-lived (for an elf) tryst without any worries about bad breakups (assuming their human partner dies of old age). Other half-races, though, either play it straight, tending to come about because of Mayfly December Romances (half-dragons—even an elf will be in their grave for millennia by the time a dragon lover dies), or in the few exceptions avert it entirely. In the case of half-orcs, it's reversed, since orcs have far shorter lifespans than humans, and are more likely to die by violence.
Actually actively averted in the Elves of Alfheim supplement for the Mystara campaign. Elven romantic relationships are not expected to be "for life" and it is rare that an elven couple doesn't drift apart anyway after at most a century. Elves with human spouses don't like when they die, of course, but don't see this trope as a big problem.
This actually a very integral part of Vampire The Masquerade. Part of what causes vampires to slowly turn more and more inhuman in attitude is the fact that any human relations a vampire has will eventually die. This either slowly drives them mad from despair or they simply stop bothering with considering humans anything more than toys or lunch. Vampires who try to fight this dissolution of humanity tend to be pretty unhappy.
Keeping ghouls (humans with vampire blood in their system) doesn't help either. They don't age, but they tend to go insane over decades or centuries for the same reason that vampires do (although slightly faster).
And if you try to go all the way and try to make your special someone a vampire like you...well, it's a crapshoot at best, but I Hate You, Vampire Dad is very prone to rearing its ugly head.
Warhammer has one. Specifically, the Vampire Count Vlad von Carstein and his beloved wife Isabella. Of course, because this is Warhammer there is precious little angst over this as they are both bloodthirsty Complete Monsters of the old school, even before Isabella became a vampire.
The Castlevania series has a couple of these. Dracula and Lisa. Alucard and Maria and Lyudmil from the radio drama, if you interpret it like that; Alucard even talks about it with him, saying, "Lyudmil… humans will return to dust eventually, but I will live on forever. Compared to eternity, my time with you is short…"
In Lunar Eternal Blue, you find that The goddess Althena averts this by giving up her immortality and living out the rest of her life as Luna. A straighter, and more platonic example is Nall, who did in fact outlive his friends, and Ruby, who has to come to grips with the fact that she will do the same. Presumably this will be an issue for Lucia and Hiro as well.
Inverted with Fire Emblem's quasi-canon couple Eliwood and Ninian. You'd think being a 1000-year-old half-dragon would mean Ninian would outlive Eliwood, but in fact abandoning the dragon homeworld makes her die within the next twenty years.
Which leads into the relationship between Eliwood's son, Roy and possible bride Sophia who, like Ninian, is half-human, half-dragon. However, there is no mention of any changes to her lifespan, possibly because she doesn't come from another dimension like Ninian does. If you also accept that Ninian is Roy's mom, then you have the interesting case of a quarter-dragon and a half-dragon. It should be noted that the support conversations between half-dragon Sophia and full dragon Fa also directly discuss this trope, albeit from the friendship angle: Fa will outlive Sophia and all the friends she has made in the game, and she's not pleased at all.
In the same series, there's also Sothe and Micaiah. It can be assumed that Micaiah, being Branded, will live much longer than Sothe. This is the reason why the two weren't together during Path of Radiance, but the topic isn't addressed much in Radiant Dawn.
Ike and Soren have the same problem. Remember that Soren's grandfather was one of only two living people to have personally met the goddess.
A point that's brought up by both Nowi and Tiki in Fire Emblem: Awakening if they form a relationship with Ricken and the player's character respectively, as both of them are Manaketes and capable of living for thousands of years. Nowi questions Richt about whether he really wants to marry her when she'd be looking youthful as ever even when he's an old man, while Tiki states that she's aware that she'll have to part with the player long before her life ends, but promises she'll remember him forever.
In Mass Effect, the prolonged lifespan of the asari has colored their relationships with most of the other species, along with their ability to mate with anything. Liara explains that asari who enter into physical and emotional relationships with members of other species do so fully aware that they will outlive their partners, and take a "long view" of the relationship.
The children from such a relationship are always asari. In fact, most asari are the products of such relationships, as there is a stigma attached to being a "pureblood".
This isn't so bad if the partner is human, turian, quarian, or batarian; with medical advancements a member of any of those species can reach 150, and drell can get up into their eighties; with krogan, who have a similar lifespan to asari, it's not even an issue. Salarians, meanwhile, are lucky if they reach 40 — because asari are so long-lived and avert Immortality Begins at Twenty, salarian parents won't even see their children reach adulthood. This is perfectly illustrated by a elderly 35 year old salarian and his 60 year old teenage asari stepdaughter in the second game. Another asari asks what species her friend's father was, is told he was a salarian, and delicately says "Oh, so you didn't get much time with him."
One asari mentions this as a reason she's hesitant to get it on with a krogan. Krogan have at least as long of a lifespan — there's mention of one having "millenia" of experience — as asari, making it a big commitment. With humans "you just stick it out a century, and they die". Shepherd is less than amused.
In one particular mission you have to get a fish for a krogan at the Citadel. In the shop, you can hear a turian and asari talking about getting a fish, and the turian said that maybe they shouldn't get a fish because they only live for a few years. This is pretty much what happens.
Asari: Well, you have to appreciate the time that you DO have with the fish-
Turian: Oh, this is the lifespan talk, isn't it! We're not having the lifespan talk!
Asari: We have to talk about it some time!
Another possible example is the relationship between Commander Shepard and Thane Krios. One of the first things that Thane tells Shepard is that he is dying of a deadly disease and has only months to live. The fact that Shepard/the player can choose to pursue this relationship anyway, fully aware of this, puts it in this category. On the other hand, it is possible that the third game may have something to say about Thane's fate. He dies from a combination of getting stabbed by Kai Leng and the final stages of his Kepral's Syndrome if he didn't die in the suicide mission.Liara can comment on this in Lair of the Shadow Broker, comparing Shepard's relationship with Thane with a typical asari/non-asari relationship, and encouraging her to treasure whatever time they have together.
Liara herself is an example of this. She is very young for an asari, but even so, Shepard will most likely only be around for around the same time as she has lived thus far. This is the asari equivalent of getting married at 20 and one partner dying at 40, while the other lives to be 150, or more like comparing the lifespan of a dog to a human.
In Lair of the Shadow Broker, she'll even mention she's only just about to turn a 109 (and typically, asari can live up to a 1000 years). Won't stop a faithful Shepard from proposing a Babies Ever After ending.
During the third game there is a fleeting mention of asari-vorcha offspring. Vorcha live twenty years if they're lucky and tend to be brutal and violent. If adopted and "socialized" they're much calmer and easier to get along with, and the premier institution devoted to "socializing" vorcha is asari-run, making it a distinct possibility that some of those pairings involved Partner Husbandry.
The main relationship in Metal Gear Solid 4 is a platonic Mayfly-December love between two best friends, one a normal man approaching middle age, and one a clone with rapidly-accelerated ageing which has left him physically seventy while being only about forty-three years old.
Played straight, averted, AND subverted in Lost Odyssey: Kaim has had dozens of wives and hundreds of children before marrying another immortal and is at the end of the game quite ready to spent the next 1000 years with her, Seth has also had numerous husbands/lovers and at least one of her children is still alive, Ming has no problem with the idea of marrying a man she can outlive by hundreds of centuries. The immortals' children have ordinary lifespans thoughh, and it is shown that, even if they learn to live with the death of their loved one, the immortals still suffer from this, sometimes even going to the place of the death of their loved one centuries later.
In the Myst series, Katran has, it appears, a normal life span. However, her husband Atrus is one quarter D'ni, and so he was still alive at 250. They were close in age. This is kind of subverted in another background novel, The Book of Ti'Ana- the lifespan discrepancy between the human Anna and the D'ni Aitrus is one of the reasons their marriage is opposed, yet the couple themselves are less bothered by the issue. However, thanks to the efforts of the villains, she ends up outliving him.
Don't forget that Shiki's lifespan is measured in "hasn't he keeled over yet"! Any romance with him is likely to end.
Actually to be fair, Shiki has at least a couple of years left, as there is an epilogue that takes place years after Tsukihime and is presumably based on the Official Couple. Also, Future Badass and all.
Though not a romantic relationship, this is strongly hinted at in Tales Of Symphonia, where the half-elf Genis Sage expresses sadness that he will greatly outlive his human True Companions.
Also, you have the option to pair off Lloyd with the half-elf Raine, who will outlive him.
Has everybody suddenly forgotten Kratos? He's a 4000-year-old angel who doesn't age, and his wife was a human woman being slowly eaten away by a mutating Cruxis Crystal. And, of course, she dies before the game even starts.
Also prominent in Tales of Phantasia, with the half-elf Arche. Even though she's from one-hundred and two years in the past, she's going to live much longer than all of her friends, and even her love interest. It's shown in one of the sequels that in a few hundred years, she still misses Chester.
Persona 3 The protagonist (official name Minato Arisato) to any of the romancable girls in all the releases of P3. By extension, the female protagonist (doesn't have an official name, but the fandom calls her Minako Arisato) to any romancable guy in the PSP release.
Again, not a romance but touched on in Touhou. Sakuya explicitly refuses to become immortal and serve Remilia forever when given the opportunity, although promises to serve her until death.
Yukari's friendship with Yuyuko was almost this, with Yukari being absurdly old even back then and Yuyuko being human. However, after Yuyuko died, had her body used to seal a youkai tree (most likely by Yukari), and became the ghost princess of the netherworld, they have continued their friendship for almost a millenium so far.
Mokou will outlive everyone but her rival Kaguya, including Keine, one of her extremely few close friends, resulting in her basically living as a hermit to avoid relationships. Kaguya herself isn't bothered by this though, both being far more self-centered and living with Eirin, who is ancient even by Gensoukyou standards.
Many doujin, such as GERANIUM by Can't_Fix_the_Helmet, which features an aged Marisa and Kourin.
In many Harvest Moon games, your character has the option to marry the Harvest Goddess, and occasionally, other supernatural figures. (In More Friends of Mineral Town, you can marry a Kappa, and in DS, you can marry a princess who's Really 700 Years Old.) While the Harvest Goddess is generally a fairly distant wife (she mostly lives in her designated spring), she's still happy to see you when you come around. There's also various incarnations of the Witch Princess in several games, who's a witch, acts like a princess, and is Really 700 Years Old. Animal Parade introduced male versions of the Witch Princess and the Harvest Goddess—the Wizard (no word on whether he's a prince or not) and the Harvest King.
"Old Man" Andrew, a supporting character from Mega Man Zero had this as a part of his backstory. He was a Reploid, she was a human, when she started aging he at one point had himself converted to look old, guess the mind must've followed suit because all he does nowadays is be by himself reminiscing.
Avoided in the backstory of Suikoden III, where it is revealed that the Flame Champion gave up the True Fire Rune, the source of his immortality, to be with his lover Sana. Played straight with Jimba, whose half of the True Water Rune grants him immortality, and is revealed to be the father of Chris Lightfellow. Nash and the vampiress Sierra. They do have the option of turning him into a vampire and who knows, maybe they already did. At the very least, he has stated that he wouldn't mind spending an eternity as vampire with her.
Played straight in Avalon Code, where Mayor Georg married a human woman (who was, in fact, the granddaughter of his first love). If the male player character chooses Georg's daughter Sylphy as a love interest, it can be assumed this trope would apply to their relationship as well, albeit not in-game.
Good Dwarf: I'll still love you when you're spoiling your grandchildren, and when they're spoiling theirs.
Hatoful Boyfriend, a Dating Sim where the human female protagonist dates uplifted pigeons, brings up the lifespan differences between humans and birds a couple of times, especially during Ryouta's route where he points out that he'd die much sooner than Hiyoko and that he doesn't want her to go through the pain of losing a loved one and being left alone like he did with his mother. It's stated in Holiday Star that pigeons live longer than fifteen years in this universe, but still not nearly as long as humans do - and Ryouta has a chronic medical condition.
In Katawa Shoujo, Hisao expects to die young from his heart condition, while none of his potential girlfriends have conditions that limit their lifespan.
The High Entia live about five times longer then the Homs do, but they have a longstanding tradition of having a Homs consort in their royal family. The same goes for the half breeds that have resulted from such unions, and you help one get together with her Homs boyfriend on a particular quest. They never really seem concerned by the differences in their longevity in either case, though.
Machina are even longer-lived than the High Entia and would be in for heart-ache if they had a relationship with someone from another race; one of them, Neonik, is 9892 years old. At least a few of them were alive during the events shown in the opening scene of the game, which explains how the world of the game came to be in the form it's in.
There are relationships between Nopons and other races. The Nopon Sage has an age of 9999 years. However, this is an anomaly as the next oldest Nopon is Yusa at 83 years and is considered to be elderly by other Nopon. It may be that the Nopon Sage's life-span was altered somehow or that he is lying or perhaps his age is due to him probably being a ghost.
In Baldur's Gate II, two of the four romanceable characters (Aerie and Viconia) are full-blooded elves, the longest-lived race around, whereas the Player Character can be of any race. Viconia the dark elf won't even get involved with a (surface) elf character due to racial prejudice. Her epilogue when she's chosen as the love interest reveals that the player character actually outlives her, as she is assassinated by her former people. Conversely, the other two possible love interests (Jaheira and Anomen) are a human and a half-elf, so a long-lived player character will outlive them.
In Neverwinter Nights, it's possible to start a romance with Lady Aribeth, who is actually dead and a petitioner spirit on the eighth level of Hell. Bringing her back to the Material Plane as a corporeal "sprit" means that she is left immortal in the sense of unaging, so in the prologue she outlives any player character, even an elf.
In the semi-revisionist fantasy webcomic Errant Story, it was fashionable for some time in the past for the immortal elves to take on human lovers, so that they can experience the mourning, sad joy of seeing their lover grow old and die (especially since relationships between two immortal elves inevitably resulted in the two getting bored with each other over the decades or centuries); apparently this was their culture's take on mono no aware. Elves, in this story, have a lot in common linguistically and culturally with Japan.
Vampire sorcerer Pat in Sorcery 101 had one such marriage in his (un)life, since he submitted to being turned to keep his wife from being killed, and has since been avoiding his only real friend because of the situation. He's in the midst of searching out the spellbooks of an ancient sorcerer who he is convinced found a magical cure for vampirism. Danny, his student (and the main character of the comic), is likely to end up in a similar situation eventually because he has a blood bond with another vampire, Seth, meaning he's stuck at his current age, health, and body shape until Seth dies. Given that Seth has lived 2000 years so far and is too much of a bastard to stop, Danny's got a long life ahead of him.
In Tales of the Questor, due to a botched Deal with the Devil centuries ago, Elves grow to full maturity in approximately 20 years—- then die shortly afterwords. One of the main characters is Sam, a fourteen-year-old HALF-elf, destined (unless a Deus Ex Machina intervenes) to die in her mid thirties. The reader may draw their own conclusions as to what this meant for her parents, and what it will mean for her. In addition Raconnans are supposed to have a lifespan of 250 years, and the author once stated that May December Romances are fairly common among them due to their long lifespans.
Edward:What...How...This isn't right! Immortal/Mortal romance is supposed to be angsty and star-cross'd and drama-ACKcough cough * gargle* chokewheeeeze...
In The Inexplicable Adventures of Bob!, Voluptua gives this as one of several reasons she cannot pursue a relationship with Bob, yet also the reason she is reluctant to leave Earth's solar system. "If I turn my head for a moment... he'll age to dust before I can look back." Shown here.
In the webcomic TwoKinds, the average lifespan of a keidran seems to be between 20 and 25. Trace (a human) briefly attempts to find a way to extend the life of his keidran lover Flora before giving up and deciding to make the most of the time they had.
In Homestuck, the fact that she's dead and has existed that way for an uncountable amount of time, however much she may look like a normal teenage girl, is why Aranea tells Jake that a relationship between them couldn't work. He maintains optimism regardless (even thinking to himself that it might work if he dies, betraying perhaps worrying faith in Death Is Cheap holding true indefinitely), but even aside from the age disparity Aranea isn't really interested in him and probably only brought it up as a more polite way of rejecting him.
Gunnerkrigg Court has Jones, who's almost as old as the planet Earth and completely invulnerable, and she's currently in a relationship with the normal human James Eglamore. She's had numerous similar relationships in the past. She also claims that she's incapable of emotion, and merely copies the behavior of humans around her to better fit in—but her lovers don't seem to mind.
In El Goonish Shive, elves are the offspring of humans and Immortals. Raven's father therefore probably had this kind of relationship with Pandora.
Played for Laughs in Mudy Quest, where Darkmoon (a drow) and Totenkopf (a gobbo) are 923 and 11, respectively. They are of comparable appearance, however; she ages slow (or is immortal), while he ages fast.
Used as a punchline in Ultra Fast Pony. Celestia is immortal, and remarkably blasé about the number of relationships she's had over the years. On the other hand, she does try to encourage her current crush (Twilight Sparkle) to become immortal as well.
Celestia: ...And that was where I met my 43rd True Love.
Completely ignored in Transformers, where romantic relationships between Transformers and human females are an uncommon but not completely rare occurrence. However, since Status Quo Is God, the relationships are never shown again after the initial episode and one might assume they've broken up again.
When Goliath and Elisa finally hook up, it looks like it'll be this. Gargoyles age at half the rate humans do, so Goliath still has about 115-120 years left in him. A bit more mild than most examples, but still, he'll outlive her by decades. Word Of God reveals Goliath will sacrifice his life for a great cause... so maybe not, after all.
Every time one of the Children of Oberon takes human form to marry (or sleep with, whatever) a human, it's like this. Anastasia and Halcyon Renard are going through this: he's elderly and terminally ill; she's a perpetually young Physical God. It's hard to say how he'd feel if he knew.
There is also baby dragon Spike's Precocious Crush on Rarity, which has shades of this given that dragons are supposed to have lifespans tremendously longer than ponies.
Johannes Heesters, the late Dutch stage and film actor that performed up to his death at the age of 108, once mentioned that the only thing he was afraid of was that his wife (who is 43 years younger) might die before him. (She didn't.)
Any Real Life romance in which one of the participants is terminally ill, and both are aware of this fact, sadly contains elements of this trope.
So do humans' relationships with most species of pet, for that matter. Inverted with turtles, tortoises, and large parrots, who can live to be in their 70's or older. Unless the owner is a child—in which case it's unlikely they'll keep the pet for its entire natural lifespan anyway—such owners have to consider what might happen if their beloved pet were to outlive them.
Hell some of those animals could outlive a couple generations of the family meaning that(for example)some kid could grow up with the same pet that their grandparent had. Obviously that is a huge commitment to make(not to mention that it is illegal to own most animals that live that long to begin with).
Per Long For This World, Aubrey de Grey and his wife Adelaide Carpenter are potentially a literal example: Aubrey is a gerontologist who plans to live forever, while Adelaide has "no interest in immortality."
Many animals, particularly insects, have large differences in life expectancies between sexes.