My daughter has been playing outside since she was 4 years old, unsupervised. She's 9 now. A lot of the other neighborhood kids have followed suit (I really suspect our "benign neglect" made the other parents thing "well if nothing bad happened to her our kids will be fine" and we have about 8-10 kids around the same age on the block that are constantly going to each other's houses and hanging out doing whatever it is they're doing (I don't really know, that's part of what makes it so great). It's almost 9pm right now, I have no idea where she is. She comes home when the street lights come on. The other day I gave her a $20 bill and she rode her bike to the Walmart nearby (no major streets on the way there). She bought her friends ice cream and was a hero. Sure there were hiccups, one time she stole the neighbors solar powered lanterns from his front yard, but we made her return them and apologize.
I guess our strategy is we just don't let her have any screens. They make her absolutely insane. So instead she leaves the house as soon as she can and goes out and plays. Even her 5 year old sister who is wheelchair bound goes with her a lot of the time, in the $1200 Thule buggy super stroller we bought (used). I guess we just lucked out, but we also insisted on it and kept letting her go out even after some woman called CPS on us for letting our kid be unsupervised. We live in a middle class neighborhood with people from dozens of countries and every major religions living together, and all our kids play together. I'm sure it's someone's mental image of the ideal childhood.