I let my DD7 stay in the house while I walk outside with DS3 (we only go to the corner and back). She is usually engrossed in something and doesn't want to stop. For 10-15 minutes, she is fine. And she knows where I am, if she needs me.
I also let her walk to the bus stop by herself in the morning (which she wants the responsibility to do). I can see her from one of the front windows, so I am not concerned. But, oddly enough, I am the only parent in the neighborhood that does this (except for a couple of 5th graders).
I think you have to give kids more responsibility as they are ready for it and ask for it.
I have a babysitter that is 12- now I only have her babysit during the day for an hour or 2 at most, and her parents live close by, but still - she is more responsible than other babysitters that I have used that are 16. I don't think there is a hard and fast rule. I'd judge on a case by case basis.
