Food is king for the youngsters.Thats how my lad started, gymnastics - he did rings, determined little stick insect that he was. He never built much mass, still looking pretty much like a stick insect, until last year though when he went at it in an organised manner and put on some muscle weight.
Had a lad at my gym worked his nuts off, stayed skinny - one of the old sweats had a word with him and said eat like a horse (clean as you can) until you feel sick for just one month and see what happens. Just needed to tip over that calorie burn threshold, which for young 'uns with silly metabolisms can be sky high.
It worked, and then some, really packed it on.