What kinds of things did you think about? Age and personality and their feelings is clearly a factor, but also I was thinking about things like ability to cross roads safely, ability to open front door with key, ability to work a telephone, ability to resolve their own fights (luckily not many in our case), can be trusted not to get hyper with freedom, can make/find own after-school snack etc. We are no-where near there yet, but mine want to know when they can be, so I'd like to begin thinking about it.
In our particular context it would be once a week, for two hours, with both parents within 10 mins drive of home and easily able to get home early if needed unexpectedly. So less hard-core latch-key than the kind I did as a kid!