I have an 11 yr old son who did the same thing. I was worried about leaving him so I called our local job and family services and asked them about leaving him. They told my that there isn't a set age though the prossicuting attorney recomends 12 yrs old but if someone would happen to file a complaint aggainst you that the childrens services does an investigation to determin what they think. They said they ask the child and the parent question to see if they think they are responsible enough. I now can leave my son for a few hours without worry because I know and he knows that he is responsible enough for this. But we do have set rules and if or when he breaks these this prevledge will be taken away.
I hope this helps