this is the problem with the whole stree/sidewalk issue. obviously we don't want a six year old bombing down 12th avenue at rush hour. but, by the same token, it's unsafe for everyone to have a 34 year old cruising a downtown sidewalk on his fully-loaded mtb.
the question is: where or when does the transition take place? age-based criteria are, in general, ridiculous. is anyone really going to be way better prepared for street riding the day after their 12th birthday than the day before? and frame size is equally pointless -- i know a lot of competent and speedy cyclists who ride teeny frames.
so, when do we start telling our children that it's time to start riding on the road, and how do we cope with making them unlearn all the things they were told about bike safety as youngsters?