Try Amazon - http://www.amazon.com/gp/redirect.html?i...
As a fellow 21-year-old-who-looks-17, it isn't the //type// of clothing you buy but how good it looks on you. You need to be following your body type and not what "college" students wear - because othewise, you may just look like a 17 year old in college clothes.
Find an outfit that flatters your body type and covers up the more childish attributes - that way they know you're a woman and not a child.
Also, go for in store buys - the things you buy on the internet might to fit the measurements they said they did and they might not look as good on you as you thought.
I'm a short person with no shoulders, so I try to wear clothes that minimizes how short my waist looks and makes me look a bit broader - a lot of my outfits are dresses that should honestly belong on those dolls that little girls have but I always get lots of compliments with them because they suit my figure.
Hope this helps!