Part of it is just old age, and body wear. People get old and look it eventually. (look at some of your classmates from age 17 and now) Then if you throw in the drugs, long hours, and harrowing schedules, hollywood crazies, etc, that add more wrinkles and wear. I've begun to notice the "jaw line" adjustment I've noticed a lot of celebrities seem to get after the 30s, At first I thought something was wrong with their faces, but then I realized it was their way of making themselves "look better." Or at least not as old. FAIL.
It's hard to face that at some point, you just won't look as "cute" as you did when you were young (and that's even if you're already ugly!). Its especially worst as an actor when so much of your career depends on your look. If you don't have it anymore, there are so many someone elses that can pick right up where you left off. But if you've lived well, that won't be all you'll have in your favor when you age, and people can enjoy your great non-looks character features even more, instead of mourning the lost of what they can see.