Very recently my oldest son has started playing outside all day, with the little girl who lives next door. He is 5, he'll be 6 in a 2 weeks and she is 7.
I try to watch them as much as I can out the window, but I have a baby inside and things to do, so it can be difficult. Her mom seems to be ok with allowing her to roam, but I do give my son specific rules. Those are that he has to stay in our yard or hers, and if they want to play in the church yard (across the street) or go in her house, he has to come tell me first. So far he's been doing fairly well with those rules, though I did catch him doing a few small things, but nothing major.
That said, he is 5, well really close to 6, but still just a "baby" in my mind. Some days I feel like I'm being stupid, being so liberal with him, and maybe it's just a disaster waiting to happen. I guess, considering my job, my mind tends to run to "worst case scenario". But then I think about my own childhood, and we were doing the exact same thing at his age. We were actually running the woods at that age! No one could even see us, we'd come in for lunch and then be gone.
I really think it's important to help kids become independent. I do not want to be a "hover parent". Then again, I don't want to be the stupid parent that let her 6 year old out to roam, and something happens, and I feel guilty forever. He is a clumsy boy too. He's had head injuries in the past couple months, one at a friends house, and one at school. But the first required gluing and the second stitches! So, you know even if he listens to everything I tell him, he's prone to injury, lol.
Well. They are outside making a treasure map right now, to search the church yard with. Exactly what I would be doing as a kid!