[Not another shrubbery]

I take it you haven't searched the forums for threads related to this? It has come up a few times before.

If your game's time-frame is short enough that character aging is not an issue, and if there are no in-game ways to artificially manipulate age, then being unaging will confer no game benefit. You could just call it a 0 point Feature in that case. If it justifies extra skills that other characters (w/o the trait) can't have, then it would be an Unusual Background.