I'm 21, and I asked the same question. The general reply was "Wait until you're 25". For the most part, I'd agree. Take a creatine,, a preworkout supplement like Jack3d, and a postworkout protein and workout right and hard, you'll fill out quick (if you haven't already done those things).
I was 150lbs my sophomore year in high school. I was a 2nd string wide receiver and wanted to get bigger. So I worked out outside of football workouts, took , a creatine, and a protein shake. I gained 50lbs by senior year and started outside linebacker. (clincher) Give it time and it'll work.