My sons now 18 and 15 started their weight training around 14 during puberty. They went to a reputable training facility and was taught proper technique after an assessment and supervised them in a small group setting. I was able to monitor them for proper technique when they did lifts at home. I have read and know of kids who hurt themselves due to poor technique and lack of supervision. They also took creatine and both ate reasonably well for teenagers and their caloric intake during school was a challenge. However their gains were ridiculous, >20 lbs in a few months.