I'm seventeen years old, but I look thirteen. Help!?

I don't know why...Maybe It's a genetics thing. I have brown hair thats a couple inches past the shoulders all one length with side-swept bangs. I don't wear a lot of make-up, just mascara and powder if I'm having a bad day. My regular wardrobe is jeans and a crew cut t-shirt. Usually I don't care about appearances, but I've actually been turned down for jobs because the boss says "I look too young" I REALLY need a job, and if it's my un-aged-ness that's the problem, I'd like to find a solution.