Generally, if your child is ready to participate in organized sports or activities such as baseball, soccer, or gymnastics, it is usually safe to start strength training.
A child's strength-training program shouldn't just be a scaled-down version of an adult's weight training regimen. A trainer who has experience in working with kids should design a program for your child and show your child the proper techniques, safety precautions, and how to properly use the equipment.
Kids as young as 7 or 8 years old can usually do strength-training activities (such as pushups and sit-ups) as long as they show some interest, can perform the exercises safely, and follow instructions. These exercises can help kids build a sense of balance, control, and awareness of their bodies.
Specific exercises should be learned without resistance. When proper techinique is mastered, small amounts of resistance (body weight, band, or weight) can be added. In general as kids get older and stronger, they can gradually increase the amount of resistance they use. A trained professional can help your child determine what the appropriate weight may be
Normally a child should not start lifting weights until he is 14-16 when his muscles start developing. If he starts to early he could suffer ligament, muscle, or even bone damage. My muscles started developing when I was 14