Probably hopelessly late, but it took a while to put this together:
Born in 1962 (i.e., same year as me), with substantial IMDB credits both before and after 1982 (i.e., the 20YO mark):
Jennifer Jason Leigh (probably pushing it -- though she appears to have worked fairly steadily since 1978, 1982's Fast Times at Ridgemont High was probably the role that made her a star)
Adam Baldwin (second lead in 1980's My Bodyguard
Tom Cruise (2 movies in 1981, including a 5th listed credit)
Kristy MacNichol
Wendy Jo Sperber
Melissa Sue Anderson
Jodie Foster
Of names I recognized....
My guess is that girls are forced ripe -- so Leonardo was looking boyish at an age where the chicks were what women in their 40s are supposed to look like.
If Hillary Duff looks like that for the rest of her career, she's golden. She's gone through all the changes she's allowed to.
Eerily Frankie Muniz doesn't look like he's going to change either. But it might be nice if he does.
Don't call it a comeback. He been here for years.
If you think you can outrhyme me, yeah boy I bet
Cause I ain't met a motherfucker who can do that yet
Trendsetter I'm better my rhymes are good
I got a gourmet plate that says I wish you would
And after seeing the xposts --
OK, I'll withdraw everyone except MacNichol and Foster. And maybe Sperber, who isn't as big a name.
In a somewhat related note, Wil Wheaton's book is now available.
Damn, someone beat me to Dean Stockwell.
Has anyone mentioned Roddy McDowell yet?
I wouldn't call Patty Duke a star anymore, but she did work steadily for 40 years or so.
And I think Elijah Wood has beaten the Frodo trap by working a lot since, in reasonably artsy stuff.
It's more impressive to me when someone remains famous after being a child star than when they start acting at 15/16 and keep going (e.g., John Cusack, Claire Danes). Plenty of people start (non-showbiz) careers in their late teens, after all...
Jason Bateman is doing fairly well now, and has worked more or less steadily since Silver Spoons.