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!
With our dd we started to consider it when she was really moaning about the alternative after school care. We began by leaving her alone for short periods of time - ie popping to the supermarket etc. We gradually extended the length of time we left her. We began in the summer so she got used to be left alone in daylight and now she is ok as the nights draw in too. We also began by letting the neighbour know we were popping out and could she knock if there was an emergency. I knew she could make a cup of tea or a basic snack with no problems. Because we increased it gradually and at random times there was never any hyperness with the sense of freedom.