I think you are looking at it the wrong way. When i was younger i was big but not huge and pretty strong. At your age you have more than likely started puberty. The "filling out" you speak of has nothing to do with age. It has everything to do with training and diet. Its the time put in to training that builds the look.

I always felt like i looked small for my strength but that was when i was finishing highschool, working out at a YMCA (completely wrong techniques), and eating whatever (fast food, snacks, crap in general). When i finaly got an understanding of training and diet, that's when my body transformed. Once you get an understanding of your body and what works with diet and training then you will achieve that filling out