Going back to the Island's resurrection of Locke......maybe when the Island resurrects you, you become immortal? That could explain why Richard Alpert doesn't age. He died, and he was resurrected. (Of course, the other obvious explanation is that he's a time traveler, and young Richard has been traveling around time.) In fact, given that we saw Locke come back to life, is there any reason why we couldn't see any/all the dead characters on the show come back to life?