This sounds like a mod you’ll have to make yourself. I think the best way to do it is to create an immortality trait, make a loot action for it, whitelist teens, young adults, or whoever, and inject its loot into the game’s age-up interaction. Either XML Injector, or Lot51 should be able to inject loot actions. I can’t remember which one.
So anytime a sim in your game does an age-up interaction, they’ll automatically get your immortal trait. I’m not sure how immortal traits work in game on a technical level, but the game already has immortal sims that you can check out.
Baby Ariel
Emit Relevart
Alex Moyer
Vampire Manifested trait
Werewolf immortality perk.
These traits all have different flavors of immortality. If you open one up and tinker with them in S4S or Lot51, then you can see what makes them tick.
