My kids have been latchkey kids since they were 8 and 9. They get off the bus and are home for about 45 minutes before we get home three days a week (the other two days one of us is working from home). The next door neighbor is generally home, the neighbor across the street works from home (and is a trained paramedic), the neighbor on the other side has both Mom and Grandma at home in the house.
We keep a password on the TV and they have rules - like having a healthy snack (and they do have a healthy snack), no one else in the house, and they are required to stay in their own yard or in the adjoining yards.
But I was babysitting siblings at nine, neighbor kids at 11, went away to college at 17 (and was taking college courses at the local university at fifteen) and trapsing through Europe by myself for six weeks at eighteen.
There are a few factors to consider:
How comfortable are you? - obviously some people are not comfortable at all with leaving kids far older than mine are home. Other people leave kids even younger than mine.
How responsible are your kids? - I have one of those really responsible kids - his sister is little less responsible, but still pretty good. Between the two of them, they don't do much wrong - and when they do they will tattle freely on each other.
What is your neighborhood like? We have a neighborhood full of people who are home when my kids get off the bus and filled with kids their age. If there is a problem, there are adults around to help.
What's your child rearing philosophy? I'm one of those parents who thinks that the sooner you start raising kids to turn into independent adults, the less chance you have of having thirty year olds living in your basement. If they are going to be ready to go off to camp, take school trips, go off to college, get married and NOT LIVE IN MY HOUSE - they should get incremental independence. Other people feel a bigger need to protect their kids from the world for a lot longer.