What do you normally wear? If you wear jeans, experiment with other sorts of slacks, maybe change colors some so you are/are not "pastel", when I think of little girls I think of pink and too much black can make me think of goth, etc. Find images you like of women in magazines? If you have long hair, consider cutting it to chin length or putting it up; I think any thought about change on your part that you like will get people to see and start to think about you in a new way?

This article on how teens can make themselves look older has a lot of good advice hidden in its principles?

http://www.wikihow.com/Look-Older-As-a-Teen