VQ, I agree in principle, if you have children you trust, but much of the gang violence and stabbings in London and general truancy and bad behaviour that is more and more common, comes from children, left without proper supervision for long periods together, getting together in gangs and, to put it mildly, getting into mischief. Some children are also inherently irresponsible. DD had a friend like that and a friend of mine had a son who was equally unable to foresee the dangers some of his actions would cause.
The problem with younger children, like 12 year olds left alone is that yes, they are responsible and reliable while everything is going OK, but they do not necessarily have the ability to deal with emergencies.
I think we sometimes overstate the extent that children are mollycoddled these days. They face dangers with traffic and parking, among other things, that we did not, which stops them playing out in the road as we did, but from 11 most children in urban areas are taking themselves to and from schools that are often some distance from home and rquiring them to catch buses, or even trains, or walk and cycle several miles.