My oldest memories of playing outside without parents are at like five years old. Schools in Sweden are usually nearby and easily reached by walking and biking so a lot of kids just go there alone or with friends from the first day. If you are unsure about the route just follow the kids in front of you lol.

When I started athletics and orienteering at 7-8 years old I would go to practise by biking this route after school. This was probably my first "leaving the neighbourhood for real" by myself on a regular basis.