It depends on so many things. My kids will be in a middle school that is not walking distance and no bus service, but I think 6th grade would be appropriate if I can fine a way to get them home.
We leave my 8 year old occasionally at home alone when we are running quick errands (never longer than an hour and never at night, and we never leave the other kids with her). She is mature and she really likes that we are starting to trust her more. My state doesn't have a law. I think the guidelines are some ridiculous age like 12.
Depends entirely on the child and their maturity at the time.
My DD is 5 months and already VERY independent (she holds her own bottle & feeds herself Mum-Mums) and Montessori-trained, so she will probably be ready any day now.
I am looking to go back to work full time and we think we would use after school care until middle school. DD1 will start middle school at just shy of 11.