It just depends on what you mean by change. Puberty allows the body to benefit from anaerobic training like sprints and so on (it does some before puberty, but the change in the body makes it vastly different after). So, I'd say perhaps 8th or 9th grade is when a child-athlete should focus in on the one or two sports they love and start training intensely for that sport. The result of which being that's when the sport should change as well, allowing more hitting and focused more on competition than learning the fundamentals (which should be taught at the earlier age).