Do you have a reference for the law being 12 in California? I'd hard that there was no age here for a child being left alone, just some vague wording about "reasonable". Maybe that's for babysitting another child?
In Arizona, I know the guidelines were up to 4 hours for 4-8 yr olds (!!!) and up to 8 hours for (I think) 8-12. I know because our upstairs neighbors worked evenings and left their tiny 5 and 8 yr old daughters alone from 3:30, when they got home from school, until 11 or so, running through the apartment complex, and after trying to bring it up with them some of us did call and report them... it just wasn't safe. OTOH, my other neighbor would leave her 6 and 7 yr olds home alone for an hour and half after school, and they seemed perfectly safe, because they knew a lot of the neighbors and knew to come to us if something went wrong, and they would otherwise stay in the house with the door bolted until mom got home... and she only worked about a mile away.
I leave my daughter (9) home alone. Now we live on a farm and I wouldn't leave her if the people living in the main farmhouse weren't home, but when we lived in the city and she was 8-turning-9 I'd leave her for an hour or so, with the protective dogs and 2 cell phone numbers. I don't think every kid that age would be safe doing this, but you have to know your kids.
Single mom to Rain (1/93) , grad student, and world traveler