How old to start?

lifting weights does NOT stunt growth. However.. being smaller, weaker and not fully developed can pose somewhat of a problem. make sure he is fully supervised and make sure his body gets plenty of rest. like... I wouldnt have him on full body workouts and the like :)
 
Well, it might do some damage if he lifts anything more than his bodyweight (I mean like weights, not body weight exercises). So if he weighs 100lbs, only bench 100lbs...stuff like that.
 
personally if it was my son i wud just have him doing body weighted exercises ( chin ups, press ups ect) until he reaches about 15/16 then styart him on a good weight training routine. if you do decide to let him start doing weights now make sure you are not pushing him and he is lifting light weights.
 
I would say 12 is too early for body building, but not for working on being fit.
Keep him active and eating right (in this case eating right means no junk, not calorie restrictions).
Learning to lift correctly (good form) with moderate weight and higher reps will be the best preparation for building later, after he has grown his adult frame.
My advice is to grow for now, build later.
 
too early. his bones and growth plates are still soft. He could actually cause permanent bone damage by lifting heavy weights.
Plus, he's a rapidly growing boy, who probably won't eat enough fo the right foods to bodybuild anyways since his body will tear through calories just to grow normally during puberty.

it would be a lot smarter for him to do calesthenics, pushups, crunches, and just get some physical activity/play some sports for a few more years. By 15, he could safely start real weight training.
 
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