A good part of that is that the kid's nervous system is continually being detuned due to growth. Consider that dog learns how to catch a ball reliably once it has reached full size.

Interestingly the myelin sheath around your peripheral nerves thins as you grow. I have always assumed this is to reduce capacitance so that signal propagation time between (say) your fingertips and brain remains roughly constant as your arm gets longer. I don't know if anyone has ever studied this though.