How can I look at least my own age or older?

I'm a 20 almost 21 year old female and I constantly get, "how are you enjoying your first year of high school?" I've been out of school almost three years and have held a prestigious job for quite sometime and still it did nothing for my look. My hair is 2in above my shoulders, I'm only 5'3, and i only wear a 32B. I look 14 all the time. I wear eyeliner but I never do it too thick. Everytime I try to wear a skirt and some heels to look older I get called a little tramp. I don't know how to look older without looking like a prostitute. Help?