Are you expecting the older child to watch the younger? Or are you just asking at what age the older can be alone (by herself). Things I'd consider: State law, age of child, maturity of child, available neighbors if there is a problem, how quickly you (or a close relative) could reach her if there is a problem, if there are others with her or not and the ages of them, safety of neighborhood/walk from bus, length of time they'd be home alone and child's wishes (not a good idea to leave her alone if she doesn't want to be).
It looks like the state recommended/mandated ages range from 6-14 (Illinois is the 14, although it is for an "unreasonable" length of time with many factors to consider), with most either 10 or 12. In my opinion, she'd be fine, assuming you did some "test drives" first, but I'm not yet a parent. At 12 I was babysitting others. I find it amazing that these days, 12 year olds are so insulated that they aren't considered old enough to be alone by themselves.