I don't think there is a magic age.
I'm from the school of "know your kid, know their abilities and limitations and assign responsibilities and privileges accordingly".
I think we've gotten away from a time when we had higher expectations for our kids. Back in the day, kids walked a mile or so to school by themselves (in all weather), did regular chores and had important responsibilities to help the family. Our way of living doesn't require our kids to step up in the ways we needed them to before, and we tend to also feel that the world is a more dangerous place.
It's not a good combination.
It sounds like your daughter was well-prepared and had been given other chances to think for herself and make good decisions. That's exactly what I would have done at that age-- or even older. Go find a place where there would be adult help if need be, and then report what's going on to the authorities.
I think it's wise to give freedom both incrementally AND when our kids are really asking for the opportunity to be responsible and independent, and when we can be relatively certain they have those skills. When I was eight, my sister and I (she was two years younger) would stay home alone while our mom went to work for a 4 hour shift. We hated the babysitter our baby brother had, and we promised to be good, so long as we didn't have to go to the sitters. While I wouldn't agree with this as a parent myself, the point of this was that all that summer-- no problems. Not one. We knew the rules and followed them. Our mother knew our limitations and abilities and trusted us to keep our end of the bargain.
All this to say: I'm not advocating leaving young children home alone, I am saying that kids are sometimes ready for more than we give them credit for.
ETA: It's interesting to read others' comments. We got to go to the mall a lot as 12 yo+ kids... my stepfather worked across the street. We had $5 for a trip to McDonalds for lunch/snacks, and just wandered around, visited the library at the mall. Never a problem.