Most Helpful Girl
you just have an older looking face. some people do, some don't. it's just the final culmination of all your features blended together. it's not one particular thing.
i'm the opposite, I'm about to turn 21. and I get mistaken for different ages all the time. this summer I got anywhere from 16-25 while I was actually 20. I have a young looking face and a mature body so I think that would explain why...if you see me up close I look younger but from far away where you can't see my face so well my body and also partially how I dress (when I'm wearing formal work clothes which is where I got mistaken for older) add some years to my appearance.
it's annoying when people think you are younger than you are, be grateful :P