I remember when I was a kid, I used to think the kid who played Dr. Zeeon the original Battlestar Galactica (his name was Patrick Stuart, but he goes by James Patrick Stuart these days for obvious reasons) was good looking. (Sorry for the bad pic–it’s hard to find pics of him, let alone good ones.)
All of You + Charlotte = beer
Marie Osmond. She was born in 1959 and The Donny and Marie Show’s last season was 1979.
For that matter, Donny is only two years older, so he was a teen when the show started. Didn’t women/girls think he was handsome?
Mr. Hinckley? Is that you?
Really? I have a nerd-crush on Xander, but it’s due to his personality, and while I don’t think Nick Brendon is ugly, I wouldn’t say he’s that handsome, either.
If we are going to mention Saved by the Bell can I mention Elizabeth Berkely?
The first person who imediately came to mind was Tuesday Weld as Thalia Menninger. The second was Tiffany Amber Thiessen. And of course both are covered in the 1st two posts. My 3rd pick also made the thread already–Valerie Bertinelli.
Stacy Dash was 30 when that show premiered. Robin Givens was merely 22 - but still too old for the thread.
Alyssa Milano.
13 at the beginning of Who’s the Boss, 20 at the end. I’d like to say I’m sorry she was in those horrible movies after that, but since I’d been praying for them for years I really can’t.
Jessica Alba was a teenager on a Flipper show of some kind and was strikingly beautiful. She’s lost too much weight in adulthood, but at the time she was gorgeous.
Chyler Leigh was and is beautiful and was on a number of TV shows before hitting 20, almost all of which lasted less than a season.
Please add yourself to the post 22 list. 
Tatania Ali who played Ashley Banks on Fresh Prince was pretty smoking.
(And I always thought Alfonso Ribeiro who played Carlton was always a lot hotter than his character.)
Applause
Jonathan Taylor Thomas from Home Improvement was pretty cute. But the problem with male child stars is that a lot of what makes them great – baby face, small stature – doesn’t translate well to adulthood, while it’s usually a plus if the girls can stay short and skinny. Plus puberty often takes more of a toll on the boys (well, at least on their skin) while making the girls/women more marketable (i.e. boob-having).
Katie Holmes in Dawson’s Creek.
My bad. You’re right.
I’ll second the nomination of “The Fresh Prince’s” Tatyana M. Ali.
Traci Lor— no, wait . . . that wasn’t TV. How one’s memories do run together . . . :o
Agreed
Linsay Lohan was so cute & striking when she started out - its sad to see recent pictures of her.
That’s assuming a successful actor has to be a classically good-looking romantic lead or action hero. Michael J. Fox (in his 20s during the run of “Family Ties,” but had been acting since his teens) would still be a popular actor if not for his medical disability. Heck, he is still a popular actor. He consistently found principal roles where being short and baby-faced was an advantage, or something that could be worked around at worst. Mickey Rooney wasn’t quite as successful as an adult actor as he was as a teen, but then again, no one is as successful as Mickey Rooney was when he was a teen. A career spanning eight decades is nothing to sneeze at. Again, he generally played roles suitable to his appearance, but so does every screen actor. I think part of the reason Doogie Patrick Harris is so likable playing characters of questionable morality is that he has retained some of his childlike features into middle age. Not that his considerable talent isn’t important too.
On the other hand, you would never know Christian Bale had been a child actor to look at him. Although not as prolific as some actors, he has been attached to a few successful projects.
I can’t list every male actor who successfully transitioned, and I’m not going to start a list of female actors who didn’t, but I’m sure we can think of a few without much effort.
I always had a thing for Kim Richards.