We live in a rural area off a highway, so no biking except in our driveway at home. We have to take our bikes to biking trails and we ride them when we go camping.
Back when we lived in a neighborhood, I let my kids ride around several blocks (they knew the limits) from the time they were 7 or so. We lived in a very safe and friendly neighborhood, though, and my kids knew a lot of the neighbors very well. They also went at least by 2's, not alone.