In quite a few of the races done in my county they do actually have an U21 catergory, at least in the league races they do. But I agree it should be more nationwide.
Then again there is also a need to sort the prize structure out in races. I have a friend who came second in a race but didn't pick up a prize, yet the 1st, 2nd and 3rd senior women who all finished in above 10th position did. The reason being because she was a vet 35 and so was the winner. Therefore the winner took the vet 35 prize then the senior ladies took the overall 1st, 2nd and 3rd female placed prizes-work that one out!