This is my daughter, Úna.
She is eleven years old and she is already an experienced bicycle commuter.
Úna rides all over town with us, in all kinds of places. Unlike many of her peers, she has a "real" bike- A lugged steel Trek Singletrack with fenders, a front basket, a rear rack, dedicated lights... for her birthday we bought her a Timbuk2 pannier bag. It was way too expensive for someone her age, but if she uses it as her book bag for the whole of middle school (which she plans to ride to), it will be a good purchase. We have started her on hauling cargo.
She has conquered the Paradise Loop, about 25 miles with hills,
and we are not sure, but it is possible she rode almost 30 miles on her 40 year old Columbia last Halloween when we brought her along for Critical Mass.
Kids are capable of so much more than we give them credit for. Úna knows her way around town, she knows where the bike routes are, she knows how to signal her turns and how to hold her own. She has no fear of the road and just takes her time.
It also wears them out so they go to bed early.