Two thoughts:
1) Growth plate fusion in males can occur as late as the 20s.
2) A young athlete experiences far more forces playing their sport than they do in the weight room.
As a strength & conditioning coach, I have no problem with weight training for even pre-teen athletes, so long as: 1) the athlete has demonstrated good technique in bodyweight squats, pushups, pullups, general athletic movements, 2) in learning those movements they have demonstrated good "coachability", and 3) they continue to demonstrate good technique, under supervision, during a slow progression of external loading.