I started Olympic and PowerLifting at 10yrs old. I went through injuries in high school and college - sports related - but my orthopedic surgeon confirmed they were driven by the extent of activity, not caused by any "damage" done from training at an early age.
There are really 2 challenges: The first is to get a kid to admit when they're on the brink between hurting and hurt, and be sure they rest and rehab the way the should. The second is to incorporate a diligent flexibility and mobility program. Stretching is boring and unsexy to teens, so it's hard to keep kids committed to it.
I don't really believe fitness has a minimum age. Our 2 1/2yr old does push ups, sit ups, squat jumps, burpees, dumbbell thrusters, and a few other movements just following along with my wife and I. He loves doing pull ups. We don't "make" him do anything, but he likes to follow along, so his increasing fitness and strength is just an added bonus. His pediatrician gives us the same advice as suggested by common sense - it doesn't hurt him, and indeed helps him, to train a bit, but his volume shouldn't be such to cause soreness, let alone pain, and the weight shouldn't be so heavy to cause him to struggle.
I've also spent quite a bit of time coaching "kids Fed" wrestling programs; the same challenges apply.
A guy has to use reason and like any athlete - listen to their body.
yeah I'm really just starting him off by getting proper form squat wise, we barely started with the bar.