I have not really looked up the best or recommended age to start lifting for kids. I did notice that Dr. Karl Nodolski has his kids lifting weights, and they are young. I would probably still concentrate more on cvardio exercises because from what I see, most kids, especially after this Covid BS, are worthless in cardiovascular exercises. About a third of the 5th graders at my school could not complete the mile under 15 minutes by either walking/jogging or running. It's pathetic. Gaining size is cool, but if they can't even do anything without getting tired, it doesn't help them much...imo. At young ages, they should be running circles and not getting so tired in sports and physical activities like they are right now. This of course does not apply to those kids who have it together, but it's what I am seeing out in the field right now. They are effin' lazy, and they complain when having to do anything physical.
I think cardiovascular exercises combined with some light strength training would be best for them.<-----opinion, not based on facts.