A character is given some means of living far beyond what they'd normally be capable of. Immortality, vampirism, cryogenic suspension or time travel flinging them to the future. But before all that, they already had a kid, or the Immortal Procreation Clause doesn't apply, afterwards. With enough time that kid can lead to a long list of generations, resulting in a distant descendant being alive at the same time as their ancestor.
Even if it's only a single generation, it's weird to see a 20-something immortal being the parent of a senior citizen. Long enough, and the long-lived or time-displaced character might have to deal with having thousands of people descended from him, unless a Single Line of Descent is at play. Being alive with your great-x times grandparent far up the family tree can be pretty strange, that's for sure.
They're quite likely to be a Supernaturally Young Parent, too.
Contrast Kid from the Future, where the descendant meets up with their ancestor before they could normally do so (or who died way before the kid was born in more distant ancestors), via time travel.
- JoJo's Bizarre Adventure plays around with this in its many story arcs. Dio Brando, the adopted brother of Jonathan Joestar and Big Bad of the first story arc Phantom Blood, is the Big Bad again in Stardust Crusaders as DIO, having become a century's old vampire whose once severed head is now attached to Jonathan's decapitated body. Because of his encounter with a mystical arrow, he gains a Stand, as do all living members of Jonathan's bloodline, which includes Jonathan's grandson Joseph Joestar (hero of Battle Tendency), Joseph's daughter Holly, her son Jotaro Kujo (the hero of Stardust Crusaders), Joseph's illegitimate son Josuke Higashikata (hero of Diamond is Unbreakablenote ), and all of DIO's illegitimate children who were conceived via Jonathan's body thereby making all of them Joestars as well: Giorno Giovanna (hero of Vento Aureo), Ungalo, Rikiel, and Donatello Versus (all villains in Stone Ocean). DIO's sons were all born in the mid 1980s, making them at least 15 years younger than Jotaro, despite all of them technically being his great-great uncles and half-brothers to George II, Jonathan's (legitmate) son and Joseph's father.
- In Mnemosyne, it is eventually revealed that all members of the Maeno family (Kouki, Teruki, and Mishio in the series) are descended from Tajimamori, the immortal former Guardian of Yggdrasil, and genetically imprinted with his desire to seek out and to protect Rin — his millennia-old (mutual) love. In the final episode, Mishio even briefly gets to meet her bloodline's progenitor.
- Tenchi Muyo!: In the OVA Tenchi Muyo! Ryo-Ohki, Washu Hakubi (who is 20,000 years old) is Ryoko's mother and a distant ancestor of Mihoshi Kuramitsu.
- Vampire Knight: Yuuki turns out to be Kaname's descendant. They end up in a relationship, though distant enough it isn't considered incestuous.
- Raizen from Yu Yu Hakusho is Yusuke's ancestor from some 40 generations ago. Being a demon, he's still alive and meets Yusuke, although he eventually dies from starvation due to his vow to no longer eat humans.
- The first Big Bad of the Charmed comics that act as the official Seasons 9 and 10 of the show is Neena, the first witch. She is the direct ancestor of all witches and warlocks, including the titular Charmed Ones. She is also one of the first members of the h. sapiens species, making her around two hundred thousand years old.
- DC Comics: Vandal Savage, who has lived since caveman times, was revealed to have many distant descendants, who he used as organ transplant donors whenever one of his body parts got damaged. One such descendant was Roy Harper, a.k.a. Arsenal.
- The Tomb of Dracula: One of the vampire hunters Dracula has to deal with is Frank Drake, who's descended from a son he had as a human. Given his old identity is Vlad the Impaler, there's over 500 years of generations between the two.
- In Wanted, Adam-One is implied to be the Biblical Adam, and is nearly immortal. He also has a considerably large number of descendants and Mr. Rictus springs his attempted coup while Adam has returned home to Africa to attend his dying son.
- In X-Men, several characters are members of or related to Clan Akkaba, making them direct descendants of the very much alive Apocalypse.
- The Belgariad: Belgarath was born 7,000 years before the main story and his twin daughters, Polgara and Beldaran, were born 4,000 years later. Unlike her immortal father and sister, Beldaran had no affinity for sorcery and therefore lived a mortal life as the wife of the Rivan King. The main story takes place 3,000 years after the birth and death of Beldaran and centres on her sixteen-year-old descendent, Garion, who is raised by Polgara after his parents are murdered shortly after he is born. While Garion thinks of Polgara as his aunt and Belgarath as his grandfather, their direct relationship is separated by hundreds of generations and thousands of years.
- Fate/strange fake: In a variant, Francois Prelati is summoned as a Caster-class "Servant" of his distant Reincarnation, Francesca, in the modern day.
- God-Emperor of Dune, Leto has many great-grandnephews at the age of over three thousand years (he had no children himself due to his transformation, but his sister had), while the latest Duncan Idaho clone is informed that his previous incarnations had numerous children... and he is talking to the descendant of one.
- In the Iron Druid Chronicles Atticus is over 2,500 years old courtesy of a youth-restoring tea only druids can craft, and at one point was the patriarch of a large clan in Africa for a few centuries. He interacted with multiple generations of his descendants and even tried to share his immortality with them. When his wife (the clan matriarch who also drank said tea) was killed and the prolonged lifespan caused his children to start going insane, he gave up on the idea and left the area for good. Presumably most of the people from that part of the world are his distant descendants.
- Kitty Norville: In Kitty Goes to Washington, Alette is a vampire who had two children before she was turned. The descendants of those children all work for her, and she loves them as her many-time-great grandchildren.
- The Lord of the Rings: Elrond and his brother were half-elves who had the choice of living as either elves or humans. Elrond chose an elven life and is therefore still alive thousands of years later during the main story. His brother chose a mortal life and became the very distant ancestor of Aragorn. Elrond is therefore Aragorn's uncle, but many hundreds of generations removed.
- In The Moomins, the Ancestor is a sort of Neanderthal Moomin who lives behind the stove.
- In Morganville Vampires, Sam is Michael's grandfather... his young adult grandfather.
- In Night World, the Redferns have a few examples due to the fact they're vampires (specifically lamia, who are born as vampires and can reproduce). Notable examples include Maya, the first vampire and mother of the family's founder Red Fern, who is around ten thousand years old, and the family patriarch, Hunter Redfern, who was born sometime prior to the 1600's, as well as his daughter Lily, who was born in the 1600's. Jez Redfern at one point comments that Hunter is her "great-great-great-great-great grandfather", which makes Lily her great-great-great-great aunt. Hunter technically counts as this to the Harman family too (who are witches), though they don't exactly mingle much. All There in the Manual also reveals that made vampire Thierry Descoudres - who was born in the Stone Age - is distantly related to the Harmans; his brother was married to their ancestress Hellewise Hearth-Woman.
- In Robots and Empire, Gladia meets a man who is her fifth generation descendant (she's 230, he's fifty). With the Spacers living for up to four centuries, this isn't actually uncommon, what is uncommon is the fact somehow being relevant and known, since it's a No Blood Ties society.
- A Song of Ice and Fire: Brynden Rivers aka Bloodraven, the enigmatic bastard son of Aegon IV Targaryen, is revealed to be still alive as the Three-Eyed Crow north of The Wall at 125 years of age. Elsewhere Stannis Baratheon and Daenerys Targaryen, his half-brother's own great-great-great grandchildren, are contesting the Iron Throne.
- In the second Sword of Truth book, Richard meets a person who, like him, is from the Rahl family, but is nearly a thousand years old (it happens in a palace where human aging is slowed down by a factor of fifteen).
- In Robert A. Heinlein's Time Enough for Love the 2,200-year-old Lazarus Long has millions of descendants, including roughly 90% of the other long-lived Howards and basically every named character from their planet, on account of his hundreds of wives and lovers over the centuries.
- Uprooted: Wizards live for centuries, so this crops up at times. Alosha nonchalantly mentions that her fellow wizard Ragostok is her great-great-grandson, but she looks no older than him and has no particular familiar affection for him.
- The Vampire Chronicles: Jesse isn't sure whether her eccentric Aunt Maharet is a relative or a family friend at first. After she learns about the Masquerade, it comes out that Maharet is a vampire and her great-grandmother by 6000 years worth of "great"s.
- Wings of Fire: When Darkstalker meets Sunny, he remarks on how she's his (many times great) niece, so he's happy to meet her.
- Andromeda: In the last season, Drago Museveni goes to the future to steal Beka's DNA to create the Nietzscheans. The crew briefly suggests using her status as "the mother of all Nietzscheans" to get them to join the Commonwealth.
- Forever: In a twist in the usual formula, following finally finding out his birth parents Abraham, the adopted son of 200 year old Dr. Henry Morgan, begins researching his linage and eventually discovers he's very distantly related to the illegitimate son of Henry's uncle. Both of them utterly overjoyed at this revelation.
- In New Amsterdam the immortal police detective protagonist investigates the murder of a mobster only to find out that the dead man was his great-grandson. He presumably had more children during his 400 year long life but did not keep track of their descendants since it was too heartbreaking. He also has regular bull sessions with Omar, a senior-citizen black Secret Keeper, who it turns out is his son.
- Torchwood: After he became immortal during his appearance on Doctor Who, this was only a matter of time for Captain Jack Harkness. In the third season, we get to see his daughter, who remarks about the weirdness of having a father that looks younger than herself. Her son, Jack's grandson, believes him to be his uncle.
- In What We Do in the Shadows, Nandor has hundreds of living direct descendants, due to having had 36 wives prior to becoming a vampire. In the first-season finale, he accidentally kills one of his descendants when he goes to visit her, not comprehending that she's very old and that the shock of seeing her distant ancestor floating outside her window might not be good for her heart.
- In the Book of Genesis, before the Flood people lived many centuries and generally started having children in their first. If you do the math in Genesis 5, Adam died after Lamech (the father of Noah) was born - seven generations later. Considering that the Flood happened when Noah was 600, this means that many of the second or at least third generation of humanity were likely still alive at the time.
- Classical Mythology: The gods are both immortal and in many cases can't keep it in their pants, resulting in them being alive at the same time as their distant descendants. For example, Zeus was both Heracles' father and great^n-grandfather on his mother's side via Io. And because Zeus' grandmother Gaia was the Earth itself, it meant Gaia and her own great-great-great granddaughter were alive at the same time (possibly four greats if you consider her husband was also her son).
- Megaman ZX: In Advent, Albert, the Big Bad, reveals to Ashe the exact reason why she's one of The Chosen Many and can use Biometal Model A. She's a distant descendant of his human family before he left them to become a cyborg, rather than being a survivor of a Maverick attack who received a blood transfusion from him. The reason she can Megamerge is because her DNA bears enough similarities to his own, and was completely unexpected.
- Fate/Grand Order: Due to the mechanics of Servant summoning — where you can summon "Heroic Spirits" of legend and history — this allows Minamoto no Yoshitsune (aka Ushiwakamaru) to meet her ancestor, Minamoto no Raikou.
- Lehran in Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn is the progenitor of Micaiah and Sanaki's bloodline, having fathered a child nearly nine hundred years before the games started. Micaiah and Sanaki's grandmother at one point addresses Lehran as "the father of her mothers''.
- In Grand Chase, Sieghart is a legendary gladiator from the ancient past who vanished while exploring an ancient ruin. He reappears in the present day and joins the Grand Chase, meeting his descendant Elesis in the process.
- Pokémon X and Y: Lysandre is said to be descended from Kalos royalty. Another character named AZ was also a king of Kalos some 3000 years ago but was granted immortality due to using a super weapon to obliterate Kalos to end a war. Somehow neither character seems to make the connection that they are related to one another.
- At one point in Ring Runner: Flight of the Sages, you visit a woman who, as Nero casually tells you inside your head, is a distant descendant of a 4 centuries old Bounty Hunter you've recently met.
- The Sims can have this happen, usually as a result of a Sim gaining extended lifespan or even immortality through various in-game items or supernatural life states. It can even reach the point that the game can no longer keep track of relations, and so one could end up dating their own great-great grandchildren.
- Due to the premise of being a Massive Multiplayer Crossover, Super Smash Bros. can have characters who are distantly related to one another. Lucina and Chrom are Marth's descendants, almost a thousand years apart. There is also Richter Belmont who fights alongside his ancestor, Simon Belmont.
- Touhou: Sanae Kochiya is very distantly descended from the frog goddess Suwako Moriya. They live together in the Moriya shrine, but because of Suwako's short stature and immortality she ends up looking like a younger relative of Sanae.
- Fate/stay night: Despite being called grandfather by Shinji and Sakura and father by Byakuya and Kariya, Zouken Matou (or rather, his real name Zolgen Makiri) has been alive for over 500 years and was the founding patriarch of the Makiri family and one of the three key mages that formulated the Holy Grail War alongside the Einzbern and the Tohsaka clans.
- El Goonish Shive: Adrian Raven is an elf, a Half-Human Hybrid with an Immortal mother. He himself has lived for centuries, and lived all that time incorrectly believing that the Immortal Procreation Clause was in effect. As one character puts it, "Raven's effectively been an ignorant man stuck in his early twenties for multiple lifetimes." Because of this, in the present day he has at least two living descendants, Susan and Diane, who are, respectively, directly descended from the offspring he had fathered in his actual twenties and his biological child.
- Girl Genius: Oggie the Jäger is the great-great-grandfather of the Storyteller. He often pesters him about great-great-great-grandkids.
- Daughter of the Lilies: Alt Text reveals that the Old Master Wu is (literally) Really 700 Years Old and that his subordinate Hikaru is, unknown to Hikaru, one of his distant descendants. Whether Wu keeps track of his family tree isn't specified.
- DuckTales (2017): In "The Secret(s) of Castle McDuck", the boys learn that their great-uncle Scrooge's parents are still alive thanks to some magical shenanigans. Keep in mind that Scrooge was a prospector during the Gold Rush.
Huey: Your parents are still alive?!
Louie: I can't believe you're still alive.
Fergus: He put a curse on us.
Scrooge: That is not fair! I very kindly rebuilt our ancestral castle for them! And I may have used some discount mystical druid stones that accidentally granted them immortality. I didn't know it would buy me an eternity's worth of criticism!
- The main character of Futurama, Philip J. Fry, is Hubert Farnsworth's many times great-uncle as well as his many times great-grandfather, given that Fry is His Own Grandpa. This is due to Fry being a Human Popsicle who was frozen for 1000 years prior to December 31st, 2999, when he was first unfrozen. He is also this to Farnsworth's young clone-son and Mom's son by Farnsworth, Igner.
- Gargoyles: The Canmores are distantly related to both Macbeth and Princess Katharine, although they've never met either of them. Wolf teamed up with his distant ancestor Hakon, though Hakon isn't technically "living". Word of God says that the New Olympians are descended from the products of unions between mortals and Oberon's Children, since Oberon's Children are immortal, the ones responsible for the Olympians are likely still alive and qualify as this trope for the New Olympians.