When I was a kid, out of necessity I biked to and from school and stayed home alone till my mother came home from work. I was 6 years old. It stayed like this for most of my childhood, but I was always a very independent child.
My sister and I were latchkey kids, from the time that I was 8 and she was 10, while our single mom worked her second job. She worked 30+ miles away and got off work at midnight. My sister and I had some... let's call them "interesting..." experiences. I know that's not what the OP is talking about here, but my own personal experience probably makes me lean toward my own kids being older rather than younger when they get left alone.
