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
it's something you can't really put your finger on. It's not like you have wrinkles or anything, there's just something in your eyes that makes you seem like you're in your early twenties rather than your teens. I rarely look my age, except I always look younger. it's a certain je any ce quoi that dictates whether young people like us get ID'd or not lol.
Maybe it's your hair. A lot of teens seem to straighten their hair so when a person leaves it natural, it makes them look more mature. The fact that you don't have bangs too could be a reason why you look more mature. If your hair was longer, maybe you would look slightly younger. In the pics where you left your hair long and straight you certainly do look much younger.
With the straight hair you still look about 20. But I can't seem to figure why.