In my opinion weight lifting shouldn t be done until 18yo unless you re a professional athlete. Chances are you will get injured sooner or later since your body is not fully developped for it. Also gaining muscular mass at a young age can cause metabolism issues later (after 30), gaining weight easier and such. Studies show it might also stunt your growth in height. Practicing sports only till about 18 then slowly moving towards weight lifting will ensure a fit&healthy body, those big biceps all kids want will come up later in life, no need to speed that up.