I would say it depends on the child. At 10, I was babysitting and left home alone. Not for long periods of time, but I still did. By the time I was 12, I could be trusted to be left alone for overnight.
The Maturity of the child plays a huge part. I have a cousin who is 18 now, and I still wouldn't trust him to be alone because he isn't mature. I was mature at 10.
I have two boys and I plan to allow them to be alone when they have reached a maturity level I feel it would be appropriate for me to leave them alone. My state does not have a legal age that children can be left alone, so it's when I say.