I don't think it's fair to consider young people in the media as role models, good or bad. Models don't really have any control over their images. A lot of them are just kids, and even when they aren't, if they don't cooperate, they don't work. Same with actresses - they have to do as they're told or they don't work, and the work environment is very stressful. The girls who end up being "good role models" are the ones with supportive families, which is just plain good luck.
If you want good role models, look to older women who have more control over their lives, and then look at the decisions they've made along the way that have helped. Also, I think women only make good role models for a particular person if they have similar temperaments and talents - otherwise you get the "well, she can do it so why can't you?" syndrome.
The only famous woman I've found who's truly similar to me is Joan of Arc, and she's probably not the best role model: dead at 19? At least she did her own thing, though, and wasn't afraid to follow her dreams.