The Cowboy Bebop episode "Sympathy for the Devil" had an immortal child named Wen whose circadian rhythms were disrupted due to the gate explosion that makes up part of the series Back Story, resulting in him not aging like he should. The vast energy of the explosion was enclosed into the gem of a ring that his latest victim, Giraffe, threatened to use to "return time to him." Spike crafts the gem into a bullet and then puts it through Wen's skull during their final battle. Wen rapidly ages to death before Spike's very eyes.
In Witch Hunter Robin, a Witch of the Week who had been alive for apparently thousands of years by feasting on others' life force crumbled into mere sand when his powers were taken from him.
Tsubaki from InuYasha had this happen when all of her yōkai were killed and her jewel shard was taken back by Naraku. In the manga, she just began to look her true age (about the same age as Kikyo if Kikyo hadn't died); in the anime, she turned into dust.
Averted (and, to an extent, inverted) in Code Geass. When V.V. loses his Code, he just dies, though that's almost certainly from the injuries sustained in a Humongous Mecha battle earlier that episode. When C.C.'s Code is temporarily sealed, she mentally regresses to the last point in her life where she was mortal — which was when she was 10 years old and living in the Dark Ages. Dialog in a later episode implies that this has happened to her before.
Used to an extreme in ×××HOLiC due to the details of the prolonged existence of the immortal in question. When said immortal dies several hundred years after they were meant to, not only does their body vanish (presumably into the scattered dust it would otherwise be by now), but almost everyone's memories of her vanish and adjust to how they would be if she had died on schedule.
Hohenheim shows the marks from transmutation all over his skin when he loses all the philosopher's stones in his body. Then he ages to what is not exactly his true age, but old age nonetheless and dies by the end of the day...ish. Traveling to Trisha's grave probably took a little while.
Similarly, once Furher Bradley's Philosopher's stone runs out, he ages rapidly.
Tsunade of Naruto has something that looks a bit like this. She's in her 50s (at least) but wears an illusion to make her appear much younger. She also focuses most of her chakra on the seal on her forehead as a reserve which she can draw on in battle to give her perfect regeneration - at the cost of increased aging. At the end of the battle she is too tired to be able to maintain her illusion, which looks like massive rapid aging - but there's no way to tell just how bad the extra aging she took was, since we don't know how old she really looked under the illusion beforehand.
Its not really an 'illusion', more of a physical transformation using ninjutsu. Tsunade's transformation takes a lot of chakra to maintain after 10-20 years of non-ninja activity, after three years of retraining she doesn't have that problem when she uses up her chakra in the battle against Madara.
In Fallen Angel, Juris, sick of being Magistrate of the magical Genius Loci city Bete Noir, passed the mantle onto his son so he could finally leave the city. Upon going beyond the city limits, he aged rapidly and ultimately crumbled to dust.
In Sonic the Hedgehog/Knuckles the Echidna, mad scientist Dimitri, after messing with a Chaos Emerald, wound up absorbing its power, and was trapped under a mountain for hundreds of years. Knuckles accidentally 'woke' him up, and Dimitri, now calling himself Enerjak, set about to conquering Knuckles' home land. However, when Mammoth Mogul came to the scene, he used the Sword of Acorns to drain all of the Chaos energy from Enerjak, and all those years definitely caught up to him. He was forced to live in an entirely robotic body just to survive, and many issues after that, only his robotic head is alive. (If you can call that 'living'.)
X-Men: Selene tends to age very rapidly when her store of life force runs low.
To explain how Nick Fury remains relatively young while being a World War II vet, it was revealed that he was dosed with an experimental "Infinity Formula" by a doctor who found the wounded Fury after he stepped on a land mine. For a while, if he stopped taking it, he'd age rapidly - not even up to his current age, but past it. After a few decades, however, his body seems to have synthesized the stuff, and he no longer has the dependency.
The 1945 Marvel Family #1 (the first team-up of all the Marvels) featured the origin story of Black Adam. He originally gained his powers from the wizard Shazam 5,000 years ago. After he gained his superpowers he decided to conquer the world and Shazam sent him into outer space 5,000 light years away. Black Adam spent the next 5,000 years traveling back to Earth at the speed of light, arriving in modern times. The Marvels tricked him into saying the word "Shazam", which changed him back into his non-powered form. Unfortunately for him his accumulated age caught up to him and he suffered from Rapid Aging, turning into a skeleton.
Averted with Gile's aunts in Buffy the Vampire Slayer. His aunts used magic to maintain their youth, but all the end of magic did was make them start aging normally again. It's still bad news, since they both made deals with demons that would make them forfeit their souls at the first sign of aging. Not long after they get their first gray hairs, a bunch of demons come knocking on their door.
Ruby Summers (daughter of Scott Summers and Emma Frost from a future timeline shown in X-Factor) keeps her youthful appearance by staying in her ruby form. While she doesn't know for sure, she thinks that if she were to return to her human form the 80 years she's spent in her ruby form would catch up to her instantly, possibly killing her. Understandably, she doesn't want to test this hypothesis.
The Far Side where "Suddenly, on a national talk show in front of millions of viewers, Dick Clark ages 200 years in 30 seconds" (which seemed a little awkwardly realistic after Clark's real-life stroke late in his life).
In the Jackie Chan Adventures fic Queen Of All Oni, when Tarakudo kills the Oni Elders in an interlude, they are so wasted (they only lived that long by trying to mimic jiangshi) that their flesh turns to dust in seconds.
Films — Animation
In the film of Howl's Moving Castle: The Witch of the Waste manages to survive her Immortality Failure, only to age, shrink, and become slightly senile.
In Tangled, Rapunzel's hair can heal and restore youth, but if said hair is cut, the person who gained youth from it begins to age rapidly. The hair's effect also wears off just by physical separation. The above-mentioned person visibly aged a decade after being away from the hair for less than a day; a week or longer might've been fatal. At the end of the movie when all of Rapnuzel's hair is cut, Mother Gothel begins to rapidly age but wraps her cloak around her body before it turns graphic. She then falls out the tower window and her body has dissolved into dust by the time it lands. She was implied to be several hundred years old.
Films — Live-Action
The League of Extraordinary Gentlemen film: The same Dorian Gray after he sees his portrait. Justified by the mechanism of his immortality: the aforesaid portrait took the effects of aging and injury for him.
In Forever Young, Mel Gibson's character was cryrogenically frozen for almost 50 years, and didn't age during that time. However, the effect wore off and he eventually aged, looking the same as he would have if he hadn't been frozen. (Which worked out well for him, since he could then be reunited with his lost love, who was now an old woman.)
In Bullet Proof Monk, at the end of each monk's tenure, when he passes the power of the scroll on to the next monk after fifty years of agelessness (and bulletproofness), he reverts to his natural age.
In The Lord of the Rings: The Fellowship of the Ring, Bilbo Baggins looks as young as he did when he first met Gandalf when he still possesses the One Ring. After leaving it behind for Frodo and traveling to Rivendell, his Ring-deferred years catch up with him and Bilbo becomes quite elderly. (In the book, there are 18 years between Bilbo's departure and our catching up to him at Rivendell; in the film it seems to be a few months.) And then he ages even more dramatically after the ring is destroyed.
Played straight in the Ishiro Honda movie Latitude Zero at the death of Lucretia.
The Hunger has a true vampire who can only provide her playthings with youth for about two hundred years; after that, time rapidly catches up with them, and they become dessicated husks. She usually keeps these husks, for sentimental reasons.
The film adaptation of She, when the title character makes the mistake of re-entering the fountain of youth.
There is a German gay porn movie named Boytropolis, which depicts a community of guys living in the jungle, minding their own business, and keeping themselves young and handsome by drinking a potion made out of plants. If they're deprived of it, they melt.
Zordon starts aging "at an accelerated rate" when Ivan Ooze breaks him out of his time-warp capsule in Mighty Morphin' Power Rangers: The Movie. Later in the film, the immortal Dulcea tells the Rangers that she too will rapidly age if she leaves her domain on the planet Phaedos.
In Vamps, Goody and Stacy become human again after killing the vampire who sired them. Stacy is relatively unchanged since she's only forty years old. Goody however, was turned in 1841. Stacy takes this badly since Goody never revealed her true age until that moment. As time quickly catches up to Goody, her friends take her out to Times Square for a last bit of fun. As Goody enjoys a baked pretzel, she reminisces about her life. As the sun rises, Goody crumbles to dust.
Near the end of The Wolverine the title character has his heightened healing factor sucked out of him. This causes him to begin aging rapidly so that within a few seconds he gains plenty of wrinkles, his hair turns grey and his eyes cloud over as if he developed severe cataracts.
Book 17 of the Lone Wolf series The Deathlord of Ixia combines this with Load-Bearing Boss. Killing the Big Bad breaks the enchantment that kept the city of Xaagon in a suspended state. The moment Lone Wolf strikes the killing blow, milennia of wear and tear catch up to the city, and Lone Wolf has to haul ass out of there. Breaking the spell also removes the permanent cloud cover over the city, allowing sunlight to warm Ixia again.
In The Picture of Dorian Gray, Dorian is kept eternally young while his picture turns hideous and ancient. When he destroys the picture, his servants find the picture of a young Dorian and a crumpled, ancient corpse on the floor, only recognizing him by his rings.
Cassius is a Denarian who surrenders his immortalizing Phlebotinum coin so the honorable Knights won't kill him. He taunts the knights by saying he need only return to the Big Bad for a replacement coin, but he never manages it. The next time Harry sees him, Cassius is an incredibly old and desperate man. In an example of Fridge Brilliance... he never manages it because the Big Bad gave his only spare coin to Dresden![[/spoiler
In Changes, the half vampires after Harry lets loose with the bloodline curse.
In Perry Rhodan, the devices providing certain main characters with immortality, poison resistance, and improved natural healing were for the longest time simple pendants usually worn on a chain; if ever lost or destroyed, the wearer had about 62 hours to live before dying of accelerated cell decay. (After a couple of millenia or so of use and a crisis involving their creator, these have been replaced by implanted chips.)
In one of the later Oz books (by Ruth Plumly Thompson, not Baum, but still canonical), Dorothy returns to the United States, and starts getting older. Luckily, it's reversed when she goes back to Oz.
H. Rider Haggard's novel She. Thousands of years ago Ayesha stepped into a pillar of fire and became immortal. At the climax she steps into it again and reverts to her true age, withering and dying.
In one early-ish Xanth book, Electra begins aging rapidly when she enters the magic-less Mundania. Incidentally, she's not actually immortal, she just slept for 700 years.
Bilbo Baggins begins to age at a somewhat accelerated pace after giving up the Ring, although he still lives for at least 20 years afterward. (In the movie, where the intervening years between Bilbo's farewell party and Frodo leaving Hobbiton are seemingly compressed to a few weeks or months, it's far more apparent.) Interestingly, Gollum, who had the Ring a lot longer than Bilbo, is still alive and mostly unchanged 75 years after losing it, probably because he was also a lot more corrupted by the Ring than Bilbo was.
Gollum comments, however, that if the Ring is destroyed, he will "die into the dust," which fits this trope exactly, aside from him not getting the chance.
It was implied that this would also happen to the 3,000 year old Nazguls upon the Ring's destruction, but this wasn't explicitly shown. In an indirect sort of way, this happened to the bearers of the elven Rings as well: the bearers are all immortal without the Rings, so we don't see them becoming older, but what happens to Elves as they age is that they tend to leave Middle-earth for Valinor... which is exactly what Elrond and Galadriel do a year later.
The first book of the Mistborn series uses this trope. The Big Bad, who has essentially been the emperor of the entire world for the last thousand years or so managed to obtain immortality by heavily abusing multiple forms of magic, essentially keeping himself young with a number of magical trinkets. When they get removed, his youth does too.
Not only are Nickolas and his wife rapidly aging without their book that has the recipe of the elixir of life (they can't rebrew it from the same process as last time because it changes every month and old recipes cause them to age faster). Also it is the standard punishment for an immortal that displeases their master Elder the have their immortality removed and quickly age to dust. John Dee's master has now threatened Dee with undoing it and just before Dee dies of age making Dee immortal at that age for the rest of eternity.
The alternate universe, somewhat more benevolent, version of Nicholas Flamel was used in the first Harry Potter novel. He had used the Philosopher's Stone for centuries as a part of the process of brewing an elixir of life, but after learning that Voldemort was seeking it out, he and his wife willingly turned it over to Dumbledore for safekeeping. The trope is played straight, though bent sideways; without the Stone, Flamel only has a limited supply of the elixir remaining. Just enough, Dumbledore says, to put his affairs in order and finish up last-minute business before he and his wife pass on. Though it's not explicitly stated, the tone of it is that he will eventually pass away quietly of natural causes, without any skin-sloughing ickyness. Fridge Logic tells us, however, that Voldemort's defeat several years down the line might've made him reconsider.
Discworld did a non-living form of this in The Colour of Magic. Time was too afraid of Bel-Shamharoth to go anywhere near its temple. After Bel-Shamharoth flees to the nether realms, the temple ages thousands of years in a matter of seconds.
The Meq subverts this by having the loss of their immortality be a vital part of their lifecycle.
Zigzagged: Aes Sedai have longer lifespans (~200-250 years) than non-channelers and remain fairly vibrant and able until they die, but develop an "ageless" look a few years after becoming Aes Sedai. Cutting them off from their powers causes them to look markedly younger fairly quickly (overnight). Turns out that they really were supposed to be living significantly (2-3x) longer, but the Oath Rod was cutting their lives short and causing the "ageless" look. Non-Aes Sedai channelers enjoyed the longer lifespan (there were confirmed cases of damane and Aiel Wise Ones over 500 and 700 years old, respectively, as they were never subject to the lifespan-reducing Oath Rod.
Played straight with Ba'alzamon, who was close to three and a half thousand years old at the time of his death. After his death, his body is described as decaying at a tremendously accelerated rate.
Oskar Matzerath from The Tin Drum was a three-year old adult who remained a child out of his own free will. Once he had enough, he underwent the same process to reverse it. His years caught up to him immediately, although he was in his twenties at the time so it wasn't as dramatic as other examples here.
Heroes has Adam Monroe, when he got his ability stolen by Arthur Petrelli. They try to explain it by the fact that, over the centuries, Adam has died and was damaged so many times, that his Healing Factor kicked into overdrive. His cells are continuously dying and recreating. So, when you remove the "recreating" part, it's clear why he suddenly crumpled into dust. It's possible that Claire will have the same problem in a few hundred years.
Episode "The Exiles": A man had his youth preserved by a skintight membrane covering his body. When it's ripped, he rapidly ages.
Also done in the episode "Death's Other Dominion": something about a certain ice world kept the survivors of an exploratory ship unaging for centuries. When one of them tries to leave.... Bonus points that effect is quite gruesome, and that he's holding Helena's hand at the time...
The Twilight Zone TOS episode "Long Live Walter Jameson": A man lives hundreds of years due to a potion of immortality. When he's shot, the effect wears off and he ages into dust in minutes.
TOS episode "The Guests": Several people are trapped in a house where Time Stands Still. If they leave, they grow old rapidly and die.
Something like this happens in one of the new series episodes: a scientist finds a way to seemingly keep people young and immortal, however it is discovered that this is done by hyperstimulating the cells of the person in question, thus causing them to eventually grow old and die in a matter of weeks.
The pilot episode of Eerie Indiana had a woman who was keeping herself and her children young forever by sealing them in bed-sized tupperware containers every night. When she was stopped, the three of them aged 30 years overnight.
In the first season finale of Sanctuary, James Watson (almost THAT John Watson) finally dies. He had lived for over one hundred years thanks to a combination of Applied Phlebotinum and a special mechanical device that kept him young, but the device finally failed and Watson died from accelerated aging. A flashback episode shows Watson being tortured by John Druitt by turning the device on and off, causing Watson great pain.
The Master does this to the Doctor by suspending his capacity to regenerate with a laser screwdriver and aging his physical appearance to that of his true age, all 900 years.
Also in "State of Decay" — when the Great Vampire is destroyed, The Three Who Rule's thousand years of unlife suddenly catches up with them...
Star Trek: The Original Series episode "Miri". Children live for hundreds of years due to a virus, but when they reach puberty they become ill and insane and die.
Hex has immortal witch Ella Dee. At one point, Azazeal foils Ella by cutting off her powers with St John's Wort. This has the effect of rendering her mortal, and, well, how many mortals do you know that are over the age of 500?
In The Hugga Bunch special, the evil queen Admira stays young by eating the fruit of the Youngberry tree at regular intervals. When the protagonists prevent her from getting the berries in time, she suddenly becomes wrinkled and eventually falls still, her skin turning ashen.
Possibly averted in the fifth season of Angel. When Hamilton shows up to replace Eve, she loses her immortality but her appearance doesn't change. Of course, we don't know how old she was.
Averted in LOST — when the immortal Richard Alpert loses his immortality after Jacob's ashes are burnt, he's perfectly fine, although he begins to age normally.
In the episode "The Tale of Many Faces", the Big Bad falls dead and turns into a skeleton when given back her original face.
"The Tale of the Captured Souls" (Season 1 Episode 7) features Peter who uses a machine to take life force from other people, and even animals, to stave off the effects of aging. The protagonist saves her parents by sabotaging his lab and giving back the life force he took from them, ultimately forcing him to spend his last remaining moments as a helpless, lonely old man.
In Stargate SG-1, while the Goa'uld cannot grant true Immortality to their hosts, they can greatly extend their lifespans through the use of a sarcophagus. This however comes with the downside of rendering most of the older Goa'uld physically dependant on them. This is best shown when Apophis went without his sarcophagus for a prolonged period as a prisoner of SG1 and his several thousand years slowly started to catch up with him, resulting in his (temporary) death.
Averted in Once Upon a Time. Emma's decision to stay in Storybrooke causes time to start moving forward, but the residents don't gain back the 28 years they spent not aging. Which means that Emma is still roughly the same age as her parents.
Averted in The Vampire Diaries when Katherine loses her Immortality after becoming human and only begins to age normally. Later however when the serum that made her human is drained from her blood, she starts to show signs of aging quickly and is estimated to die from old age in a few months.
Sir Arthur Conan Doyle's The Lost World has a Spanish woman named Anna, who discovered the Fountain of Youth and remained young as long as she applied some of the water to her skin. This act caused a drought in the nearby area. She starts to age quickly when the water runs out (she ages fifty years over the course of a day), but when the water dries up at the source, she immediately crumbles to dust.
Myths & Legend
In (some) folklore, usually what happens to vampires if you manage to actually kill them.
In the Japanese folktale of Urashima Tarō, the title character returns home after living for only three days in Ryūgū-jō, the undersea kingdom of the Dragon God Ryūjin. Turns out a long time has passed in Japan and everyone he knew is now dead because of a tremendous time-differential in the undersea kingdom. He decides he has nothing left to lose and opens the box which he had been told to never open. Causing him to disintegrate into dust as 300 years worth of age instantly catch up with him.
This is also a recurring theme in the Western fairytales where the protagonist is spirited away into the Fairyland for centuries without realizing or feeling the passage of time. Sometimes they would return to the human world, only to discover that in the meantime year, decades, or even centuries had passed while they hadn't aged. Upon their return, all that missed time would catch up with the victim spectacularly.
In one fairy tale, a human newly returned from fairlyand has to actually touch the ground for the aging to kick in, presumably because otherwise he hasn't really returned to the mortal world yet. His fairy wife/girlfriend/whatever lets him return on a horse, warning him not to dismount; inevitably something makes him fall off, of course.
The Irish tale of Oisin is like this. He is the son of the Irish hero Finn McCool and he falls in love with a fairy, who takes him away to Tir na nÓg where each year inside lasts a century outside. When he decides to return to Ireland after three years he is given a horse and told not to touch the ground. 300 years have passed in Ireland and it is now a Christian country. He sees a man trying to lift a stone to build a road and offers to help but he falls of the horse and is transformed into an elderly man. In some versions of the tale he meets St Patrick before dying.
This is why King Herla of The Wild Hunt can't get off his horse. He spent a few centuries at a fairy wedding party. As soon as he gets off of his horse (given to him by the fairies) and sets foot back on the earth of the mortal world, time will catch up and he'll age to death in a matter of seconds.
The game makes it clear that the younger the vampire, the more... meaty the remains. Elder vampires just turn to dust when they die.
Additionally, Ghouls (humans fed on vampire blood and granted some of its power) do not age as long as they get a dose of vitae once a month. If they miss it however, they rapidly age-up until they're where they should be.
While the spell "Polymorph Any Object" could conceivably be used for immortality cheese, it is vulnerable to being dispelled, leaving them at either the age they started the cheese, or their actual age, depending on the DM.
The plane of Thelanis in Eberron has a timeflow one-seventh that of the Material Plane. However, when a visitor returns to the Material Plane, that time catches up with them...
The Longevity potion reduced the drinker's age by 1-10 years. However, each time one was drunk there was a 1% cumulative chance that the effect of all previous potions would be reversed. If a 150 year old human had drunk many such potions and was effectively 50 years old when this occurred, they would suddenly become their true age and possibly die immediately of old age. Old age being one of the few irreversible ways of dying in D&D, this is a very serious threat.
Mayfair Games' Role Aids supplement Dark Folk. Any creature that drinks from the Well of Life will cease aging for as long as the drinking continues. If 1 week passes without drinking, the creature will suddenly become its true age, possibly resulting in death by old age.
One of the plots for a All Flesh Must Be Eaten campaign has an evil Chinese alchemist who found the formula for immortality. Some of the rare herbs just went extinct and he's looking for replacements, but he has to hurry because he's aging a year each month.
Cthulhu Companion adventure "The Secret of Castronegro". Bernardo Diaz has lived for 300 years due to the ruby ring he wears. If it's removed from his finger, he will instantly die and his body will shrivel.
The Fungi from Yuggoth adventure "By the Bay Part I". Lang Fu's Coat of Life has allowed him to live for centuries. If it is ever removed for more than a few minutes, his body will begin an irreversible aging process that will cause his rapid death.
Masks of Nyarlathotep campaign, chapter 3 "Egypt". Omar Shakti, the high priest of the Brotherhood of the Black Pharaoh, is several thousand years old. If the investigators kill him, his body will immediately crumble to dust.
Supplement Demons of the Burning Night. While wearing the Helm of Kadaena the wearer accumulates 10 years of aging during each combat, but the Helm prevents the aging from taking effect. If the Helm is ever removed all of the aging immediately takes effect.
Supplement Jaiman: Land of Twilight, adventure "Cult of the Third Moon". The Priestess and Sisters (acolytes) of the title cult have remained young for 150 years by Vampiric Draining the Life Energy from sacrificial victims. If the Priestess' amulet is removed she will suffer Rapid Aging, become her true age and die. If the amulet is destroyed, so will the Sisters.
Briefly appears in Mother 3, afflicting the protective vines around Chupichupoyoi Temple.
As Ōkami takes place in a Fantasy Kitchen Sink of Japanese myths and legends, it's unsurprising that Urashima shows up. However, this time his Immortality Failure results in a happy ending, as he is now once again the same age as his wife.
It's been suggested, but not confirmed, that a weird or mild form of this is happening to the night elves. According to the lore, they are naturally long-lived on the order of a lifespan of a few hundred years, they were magically granted immortality ten thousand years ago, and it was revoked less than a decade ago. After less than a decade, some are starting to show signs of age beyond where they left off; for example, Tyrande Whisperwind (leader of the night elves) was a young adult at the time of being granted immortality. In the game right now, she's developed fairly noticeable crow's feet, even though if she were to continue aging normally from where she left off, that shouldn't happen for centuries. It's left vague whether this is age catching up to the elves or simply the effects of stress(which they are under a lot of), however.
Aegwynn is in a similar boat, using her powers as Guardian to extend her life for more than 800 years. After expending most of her magic (read: having it forcibly ripped from her body by her own son) some thirty-odd years prior to Vanilla World of Warcraft, she no longer has the power to keep herself young, and has been steadily aging as a result. Though the comics would show otherwise... This is because, after her banishment, she comes back to use her remaining magic to resurrect her son (after he is decapitated by his own apprentice), who, in turn, used his powers to give his mother proper retirement. She is given a hidden house in a valley in Kalimdor, protected by multiple wards (one of which keeps her healthy) and thunder lizards. Then Jaina Proudmoore decides to settle the valley...
In a Nightmare Fuel moment for the Captain Marvel cartoon, the villain Black Adam was tricked into saying "Shazam," and reverted to his "mortal form." He hadn't previously assumed that form in several thousand years, being a native of ancient Egypt. He crumbled instantly into dust. An unexpected Family-Unfriendly Death, made worse by the normally Technical Pacifist heroes manipulating him into killing himself. (To be fair to the heroes, they probably didn't know de-powering him would have this effect.) This actually happened in the comic book too.
Also happened in the direct-to-DVD crossover with Superman, only it wasn't Captain Marvel who made Black Adam transform back. Instead, he did it willingly, choosing to die rather than spend a hundred-thousand years in the farthest reaches of the universe.
In the Samurai Jack episode "Jack and the Lava Monster," Jack defeats a warrior who had been immortal for thousands of years as a punishment from Aku. He immediately ages to look about 80, but that's still pretty spry for someone in their thousands.
** Especially since, oftentimes, the heroes were one step ahead of Kortan's forces. A few times, the Jettators (former immortals) who had just given their knowledge and power to Quentin McLeod would be forced to face Kortan himself in combat. Not only are they too weak to defeat Kortan, but said magic ritual also means that any Jettator who breaks his oath (i.e. fights another immortal) is destined to die. Their swords being shattered by the ritual probably didn't help either.
A very sad case in Adventure Time. In the episode "Betty", Ice King becomes Simon again after an Anti-Magic abomination drains his crown's powers. Unfortunately, the crown is also his Immortality Inducer, and time quickly catches up with him. In the end, Betty defeats the villain of the episode to return the crown's magic, turning Simon into Ice King again.