Is there a trait (even from a mod) that prevents one sim (played or not) from aging?

You can do this with MCCC. Just flag that sim as immortal and they won't age.

If you want the sim to be able to die, but not age, don't flag them as Immortal with MCCC. Flag them for no aging. No aging prevents the sim from going up to the next age category, immortal prevents them from dieing at all, from any cause.

Get them potions of youth when they get close to a birthday (rewards store)
Have a spellcaster make an Immortality potion for them. There used to be a bug with this where if you rotated away to a different household, the "immortal" sim would die, but it is reported fixed.