I think I was 11. I'd had an adult-size Columbia for several years, however, since my feet could barely touch the pedals, self-sustained riding took until then. There were never any training wheels.
My daughter just turned 8. We have taken off the training wheels on her bike and she can keep it going for about seven seconds. I expect she will be able to ride unassisted sometime this year.
Our school has "Wheels Day" periodically -- kids bring human powered device to school. Through second grade, most kids brought tricycles or scooters, so I'm not sure that the majority are able to ride bikes by then.
I started with a tricycle -- one of the classic, all-metal ones. I soon learned that I could take it to the top of a steep hill and get into an aero crouch, standing on the back platform with my chin on the seat. I'd regularly pass cars. Braking was quite effective, but a pair of sneakers lasted only a week or so.
Paul