1) 15 is a perfectly good time to lift...hell I started lifting seriously when I was in 6th grade. The main thing is to lift properly and don't over do it. It's like running, if you decide you want to run you don't go out and do a marathon that day you build up to it. Set weight lifting goals and work towards them.

By the way I'm 6'1 240...if it stunted my growth I can't tell...I'm bigger than average.

2) Hell it doesn't matter what your weight looks like on you if you are boxing...if you are a body builder or a model then it may matter but shit 150 looking like 130 sounds fine to me...I wish I looked 220