I got a thought. We never see Richard Alpert age.. EVER. He's been there since at least the 50s. Juliet says he is really old. I think that the island has some sort of healing factor to it. If you die, you die. But if you are injured, you heal quickly. It also results in once you reach maturity, you stop aging. Ben has been on and off the island a lot, so he has aged a bit since he was a kid. Aaron aged normally because he was a baby. Walt had slowed aging on the island and grew quickly once he was off (hence him looking so much older in the span of a few months off the island).