Also make sure you have enough Androgen Receptors for the dht to connect to, at your age I would assume you should have an adequate amount of AR and hormones to flood your penis with every time you get an erection, so if your just looking to gain length I would suggest that you whack it for like 30 minutes or so then after you pop start to do some length work but if your looking for girth gains do some edging session's ( edging is like whacking off but instead of you blowing your load you re frame from blowing one) then do some girth work, Hope this helps.