Muscle growth is accelerated with the onset of puberty, around age 13-18 years old. Resistance training is key when the goal is to increase muscle mass, size and strength. This can in fact increase muscle weight by up to 15 percent per year during these years. A general strengthening program should have an adequate warm up and cool down and address all major muscle groups and avoid sports-related injuries. It’s important to train with focus and intensity, not just go through the motions. For increase in muscle size: do multiple sets of 8-15 repetitions/set. For increase in power/strength: lift heavier weights and do multiple sets of 4-6 reps/set. Training should always be done under adult supervision by a certified professional.