Anyone with a small child around is likely to have figured this one out for themselves.
Apparently, there is a neurological basis as to why young brains are better at learning new things than old ones.
My experience of learning Japanese certainly bears this out. I spend a fortune, countless hours and am barely able to string a sentence together. The Boy watches TV, chats with his mum and is, as far as one can be said to be at his age, fluent. Go figure.
So it would seem learning, like youth, is also wasted on the young.