How to look older for my age... I'm 20?

I'm a male, almost 20, and I look very young for my age. I'm 5'6", not super short, and just went from 105 lbs to 125 lbs on a weight gaining diet. Most people would guess I look 13, and if it wasn't for a midget in my senior cl of high school, I would have definitely been voted "Most likely to be mistaken for a freshman" by a longshot. I'm going into sopre year of college now and look very immature for my age still. I work at a courthouse and one guy asks how old I am. When I tell him I'm 19 he didn't believe me. I dress mature too, have mature hair, and even walk maturely, so what else can I do to look more mature for my age. TIA