How old....

If the child is mature enough, I would say early teens, maybe in some bases 11 or 12. I persoannly was allowed/forced to stay home at an early age and I did not have any problems while alone. Was it a good thing to do, probably not but I think a child can be left alone without issue as long as they are atleast 10.

The answer to this is not simple, it depends very much on the kid and the environment. Some kids are naturally more responsible and can be trusted to handle things at a much younger age. I know kids who could easily stay by themselves at age 8. On the other hand I know some teenagers I would trust to know the phone number for 911. :) As to the environment: if you live in a safe neighborhood or have friends or relatives that live nearby in case of emergency, then I would again say about 8 for the min age.