I found that really interesting too. I guess it really comes down to the child. I leave my 10 year old on occassion for an hour or so to pick my younger two up at school, run a quick errand, etc. I am never more than a couple miles away, I call our home phone from my cell phone so all he has to do is hit redial if he wants to call me, and there are always several neighbors home if he felt he needed someone quicker than I could get there. And he knows not to answer the phone unless it is me (he can see the caller ID) and not to answer the door.
That said, I am not sure my younger son will EVER be mature enough to be trusted home alone! He takes impulsive and lack of good judgement to a whole new level!
I am really curious what others have to say. I think it does come down to the child and probably where you live as well, but 12 in general seems quite old. I was staying alone for short periods by age 8 and babysitting by 12.