where i work
we have a general rule that kids 6-9 can only go on cardio equipment, kids 10-11 can use some of the safe leg press machines and the cardio rowing machine, kids 12-14 can use all machines but no free weights. 15+ can use everything. for the kids 10-14, who are allowed to use some machines for weightlifting, we tell them they must use a weight they can get at least 15reps with. if they cant get 15 reps they need to lower the weight. i think there is a little room for exceptions to be made but for the most part this is what we stick to.
here's a funny story though, while i was working one day this kids walks up to me and asks if i can show him how to work out. he was 7 years old. i had seen him doing karate a few times and was really impressed at how quardinated and powerful he was...despite weighing 55lbs. so i asked him what he wanted me to show him and he walks me over to a incline chest press machine. since his range of motion was so small, i set the machine to make the first initial part of the lift the hardest(because thats where his range of motion would be). he put it on 40lbs and did 50 of them. my jaw dropped. he had perfect form, and did not seem tired at all. then he asked me to show him the lat pulldown so i took him over there and we put it on 30lbs and he did 100 of them, again with perfect form. of course the wheels started turning and i got to thinking i could have a world class athlete on my hands.LOL next we went to tricep extentions with the ropes...and again he floors me and puts 25lbs on and does 100 of them. it happened with every exercise we did, pullups (12 reps), dumbell curls (12lbs 25reps), db bench press (15lbs 33 reps). so i started asking him who taught him how to work out, and he said he just looked at his dads magazine. so iasked about his dad and come to find out he was a professional football player and now coaches somewhere (dont know where). his mom came up a few minutes later and told him its time to go so he goes and grabs his bag and takes off his karate gi top and puts on a t shirt. when he took off his shirt it confirmed everything i was thinking about him...at 7 years old, he had the beginings of an amazing physique...triceps, pecs, delts and upper back we all starting to develop. it was one of the weirdest things i ever saw. his name was calvin, and i only saw him a few more times, but man the kid has some crazy genetics!