Why do we need to teach kids starting at a young age?  

1) Teach them proper mechanics of human movement in all domains, early,  before “bad habits/movements” are learned.

2) Keeps them safe and significantly lowers the risk of injury.

3) They learn to be more effective and efficient which will transfer over to any sport or life activity they pursue.

Here is Kelly Brown of CrossFit Agoge and CrossFit Kids Training staff explaining the importance of the shoulder and hip in human movement or performance.