Juniors improve faster not just with good coaching, but also because they grow and get stronger. So naturally the older they get, they will be faster and stronger making them better players quicker than an adult. Adult players are already fully grown so thier improvement is limited to what they can learn technically and tactically.

It's down to neural plasticity. As an adult your neural pathways are fairly well established and they have less 'plasticity', so it is harder for and old dog to learn new tricks.

In children, the brain can alter the body's response to an external stimulus i.e. a moving ball much more quickly.