what age is the magic trustworthy age?

what is the golden number when children can be left home alone to fend for themselves? I have never left my 9 year old home alone for any amount of time in spite of the fact I think it would be okay and at that age I was babysitting home alone in someone elses home and responsible for their children no less. I believe there are laws now about that sort of thing, there might have been way back when too but no one paid much attention as far as I could see. We were not exactly latchkey kids growing up my grandmother lived in a mother-in-law apartment attached to our house and she was in residence most of the time but not all of the time and in the mornings my brother and I were responsible enough to get ourselves fed, dressed, lunches made etc...not once did we set the house on fire, let in a stranger or miss our ride to school. I do recall more than one sick day being left home alone to fend for myself. Again nothing earth shattering occurred. I am bemused by the David's Mom as she will not leave my 15 yr old nephew (Z) home alone even for the 1/2 hour it is taking her to pick up David. Z has given her absolutely no reason not to trust him for at the very least a 1/2 hour all by his lonesome never mind those time during the school year we get him overnight because no one would be home till 10 at night. I can't imagine that at 15 we won't be able to trust C home alone although knowing his father's and other various and sundry uncle's past histories he might not be left alone till he's 35
I should add MIL is particular about who is or is not in her house anyway so the fact she won't leave Z is not so very strange. It still amazes me after all these years we have finally been allowed a spare key to her place.