As a 14 year old, I think around 13 is a more appropriate age to ride on the street, at least for an average kid. After being hit while riding down the sidewalk, on the wrong side of the road, it was a wake up call. Now, I am very careful, and have avoided many close calls since I started riding in the street. Just a couple weeks ago, a car, after blowing the horn, veered into the bike lane, (one of few in the city), and ran my friend and I into the curb. I can't really see my 12 year old sister knowing how to react in a situation like that.