This happens to Fern in the Grand Finale of Adventure Time. After Finn helps Fern to kill the Grass Demon, his body starts to deteriorate since the Grass Demon was always part of him since his creation. He manages to survive long enough to help Princess Bubblegum and her army to rescue Finn and Simon from GOLB before he dies, leaving only a seed in the shape of the Finn Sword behind. Finn uses the seed to plant a new tree with a restored Finn Sword around the remains of the destroyed treehouse.
Arcane: After Viktor collapses in his lab, it quickly becomes apparent that he's not long for this world. This drives him and Jayce to desperate methods seeking a cure.
Played with in an episode of BoJack Horseman when BoJack tells Sarah Lynn, his pseudo-daughter of around 30, that she can't stay with him anymore and suggests she go to rehab. The exchange goes a little something like this:
BoJack: I'm sorry Sarah Lynn, but you can't stay here anymore. Sarah Lynn: Okay. I'll just text one of the billion people who will let me party at their house. BoJack: Well you should... not... do that. Sarah Lynn: Oh, I know. I know, but I'm at a point in my life where I don't have to 'grow as a person' or 'rise to any occasions,' so I can just keep surrounding myself with sycophants and enablers until I die tragically young. BoJack: Wh–what? Sarah Lynn: Yeah, it's pretty much too late for me.
Played straight with Herb, as we know he's going to die when we first meet him and it's revealed he has about six months to live. He gets his affairs in order. He asks Sarah Lynn to publish his manuscript after he dies, wanting his legacy to be something other than Horsin' Around (which she doesn't – not just because she was so high that she forgot, but also because everyone agreed it was horrible). His funeral episode reveals that he actually went into remission, however... The reason he died was that he was tweeting while driving and ran into a truck that was full of peanuts, of which he was allergic to.
Also played straight with Beatrice Horseman in Season 4, as she develops senile dementia and the doctors tell her son that she hasn't much time, if any, left. She dies not long before the events of Season 5 Free Churro.
The Dragon Prince: When Claudia resurrects Viren with dark magic in Season 4, his revival will only last for exactly 30 days. That's how long they have to free Aaravos, who claims he can restore Viren's life permanently.
This is a major plot point in the Futurama film "Bender's Big Score". Lars, despite being in love with Leela, breaks up with her because he is actually a time paradox-created clone of Fry who went back in time and lived an additional 12 years in the past before returning to the 3000s. After learning from Professor Farnsworth that all the time paradox clones will die in order to correct history, he ends his relationship because he wants her to be spared the grief of his eventual death. It comes to pass at the end of the movie when he dies via Heroic Sacrifice.
One Hey Arnold! episode has Grandpa Phil dreading his 81st birthday since his family is "cursed" to die at that exact age. Even though a doctor's check-up shows that he's capable of bench-pressing more than 200lb. he constantly talks about how he's doomed to die. The episode ends with Arnold pointing out that his relatives all died when they were 91, meaning that Phil's at least got a good 10 years before he needs to start worrying.
On an episode of The Jetsons, George goes for his physical and, afterwards, the doctor tells him he's probably going to die soon and says if he has anything he's ever wanted to do, do it now. George takes advantage of this to finally tell off Mr. Spacely. Spacely is impressed enough by this display of fearlessness that he offers George the opportunity to be the test pilot for the indestructible jacket his company has designed. Just before the last test (where two missiles will be fired at him), the doctor tells him there was a mistake (the result of the diagnostic probe accidentally going into an ancient mummy, rather than George's body) and he'll probably live to be 150.
In an episode of The Venture Bros., Dr. Orpheus is accidentally shot by Action Man. When everything is resolved, he grabs his hands...
Dr. Orpheus: Two years, seventeen days. Action Man: ...what? Dr. Orpheus: From a stroke. Good day, sir!
This eventually comes to pass in Season 7, but he ends up in a coma after Orpheus and the others manage to get him to a hospital in time.
