I think it very much depends on the child.
I stayed home alone regularly the summer I was 10. My younger sisters were in daycare, and I was in bicycling distance of my mom's work, which was across the street from the local library, while our house was 2 blocks from a city park and the municipal pool, to which I had a summer pass.
My sisters, however, weren't allowed to stay home alone until about 13 or 14. As my mom put it, she did worry that the house might burn down around me and I wouldn't notice because I had my nose in a book, but with my sisters she was more concerned that they would start the fire.

I also think where you live plays a big part. We lived in a small town. Big city life is a bit different.