Here in Chicago the law is 12 and under are allowed on sidewalks. Older than that and you are technically breaking the law -- although tons of full-grown adults ride the sidewalks. I even ride on them with my 7 year old daughter. We slow down tremendously around pedestrians and people don't seem to mind our presence.
My father lives in a small town in Wisconsin ... there I am fine with her on the street with me even at age 7. But, that town is so small it doesn't even have a single traffic light and there are several Amish farmers in the county so horse-pulled buggies are not uncommon on the roads. Different atmosphere.