I honestly believe the best thing for kids are calesthenics and body weight exercises (pushups, situps, dips, chins, etc.). With all those hormones running in their system as teenagers they are going to grow no matter what. I don't think I want my son to touch a weight until he's 18. I think he can get so much more from bodyweight exercises. As far as kids under 12 they should be doing what kids do; going outside playing hide and seek, tag, kick the can, running bases, things of that nature. But unfortunately these kids now all they want to do is sit in front of the xbox or ps3 all day. Sad