It just came into my head as I was thinking about the meaning of using the right muscles to play in tennis. The answer is probably as simple as we are adults and we are much stronger than kids. That means our arm is strong enough to overcome the ball. On the other hand, a 5 years old kid probably find playing tennis is more like hitting a medicine ball, so he needs to find a way to overcome this. Then the kid is forced to use the entire body or fail. The kid retains what he learns, so going into adult hood he throws his whole body into the ball. I dunno, maybe a good way to understand how to hit with the body is to imagine what would you do if you were playing tennis with a medicine ball.