Ummmm... when everyone's dead and gone and only small children remain?
Seriously, though. I think when they are old enough to know better than to let strangers intice them into a car or be able to at least run away when approached, along with being old enough to be left at home alone, and they know the rules of the road, then... that's when they can bike alone. I don't know what that age is. I guess it depends on when the parents teach their children better.
Koffee