Really does depend on how mature and responsible the child is. Both my parents worked full time jobs when I was young, and they often left me and my sister home alone. We were both Preteens.we never had any problems or mishaps, we knew what to do in an emergency, had all the important contact numbers, and knew our neighbors. I do not remember a single problem that we had. But then I guess technically I was not home alone since there were two of us :)
In my opinion it made us more independent, responsible kids. we were proud that our parents trusted us and never took advantage of their faith in us. No parties or having friends over without first telling them. We did not do any silly things. we waited for them to come home before we got into trouble, we were still kids after all.
By the time we hit our teens, we were much more responsible then most of the kids in our class. I always figured it was because of the way our parents treated us.