What age is safe to start lifting?

I would suggest talking to your pediatrician. They usually know more about your kid than anyone else, physically. If you think about it as lifting tightening your muscles, and stretching well stretching your muscles, you may want to consider doing a type of exercise that stretches the muscles (to facilitate expansion later) during puberty. Exercise like this would be anything like swimming, running, or a martial art. Most boys have 3 growth spurts, I would definitely be skeptical of starting a full weight program until the end of the second one.

Tight muscles around growth joints (ie knees) lead to multiple problems...especially when those joints start growing and the muscles around them are too tight to allow for proper expansion. I am not a doctor, nor did I stay at a Holiday Inn Express last night; however, my cousin and I are both larger men and he started working out (with a full free-weight program) at age 13 and me at age 15 and his knees have been in much worse shape than mine, and our doctor said this was the reason why. Granted, he was stronger than I was in high school; however, long-term...I think I am better off.

Stretching should be a part of your workout regardless of your age (Wish I had someone imprint on me the importance of good flexibility back when I first started lifting LOL)

Tight muscles around the joints (Knees) are not the problem so much as it is week tendons. My advise would be to get you kids off the couch and outside playing tag riding a bike ect. and when they start lifting weights focus on low weight high rep until they get there forum down and there base built (there is no need to rush).

Some kids have already reached their full potential in growth by the age of 15... but some do not. I started when I was 15... but their is really not an age on it. If you feel like your kid is almost ready get him doing things like push-ups and core training first.. then for a year or so do light weight in the 12 to 20 rep range. Things like flat bench.. shoulder presses.. squats... dead lifts for a while then add in other exercises as you see strength gains.