Whether or not weight lifting/strength development is cost effective at an early age depends on the build of the kid. My kids were small until late power bursts in growth. They did light weight exercises. Their mother, a personal trainer supervised. Freshman year and 5'11" 135 the only thing my son was going to pull was a bone. His mother focused more on nutrition with the weight lifting. Soph year he was 6' , 160. Part was natural growth. Part was nutrition. Each year from nine on people would ask how my son was trained to throw hard. My response: We fed him.