My DS has been playing out since we moved to this house when he was nearly 6. It totally depends on the set up though - if we had stayed in our old house on a long road where we didn’t know anyone, and there was lots of traffic - probably I would t have let him out until he was 10.
We are in a short row of 12 houses, the only cars are home owners, taxis and delivery trucks, there are a group of kids who play together - the kids all have a circuit around the houses, with very clear boundaries - and if they go out side the area they aren’t allowed out the next day. We are at one end of the row so by sticking my head out of an upstairs window I can always see them.
It’s wonderful, the freedom they have is incredible, last summer, five kids spent about 8 hours a day playing dens inside a beech hedge - the parents just nipped out every now again with food! They raid garages (with permission) and built an amazing cardboard fort complex. They have invented their own versions of hide and seek and capture the flag, and they are working out rules of play, and tolerance and all sorts of awesome stuff that you only get from undirected play.
However, I also have a just 4 year old and only let her play where I can see and hear her from the house - we are at the end so that’s quite a wide area, but I always need to keep focused on her even if I’m inside doing chores.
And, if there was water / ponds anywhere, I don’t think I would let even my now 7 year old out alone unsupervised near open water...