With close supervision, I feel that a child can begin training with weights at a very early age. Why are the teenage years viewed as the optimal years to begin weight training? Growth plates are not closed at this stage in adolescent development, just as they are not closed when a child is 7 or 8 years old. After a strong core is established in a child using pushups, situps, chins, even BW squats, I see no reason why a young, healthy child cannot then start training with squats, deadlifts, etc. I'm not talking about going for heavy 1RM, but basic strength training. I really believe the key is being taught correct form and a watchful eye when they train. Make sure they are using weight light enough that they can use proper form before trying to add any weight to the bar.