I have a 4 year old, in my mid 40s, and think about this a lot for some reason. In the 70s during the summer, when I was around 7, we would wake up, eat breakfast, go out and about wherever we wanted, come back for lunch, out again, come back for dinner, and then out again past dark to play flashlight tag. I think I would be charged with child endangerment these days if my kid (3 years in the future) was doing stuff…
This seems to run rampart in the US, but also a little bit over here in Germany.
My daughter (now 12) is allowed to use public transport across my city (Munich) since she was 9 (close to 10) - and my son who's 9 will change schools in summer and take public transport to there as well.
What's (still!) normal here might be child endangering in the US. The kids are free to roam from here to the city's river Isar (around a 20 minute walk, 1 subway station) because there's just a green, nice place.
And no, Munich is not a rural area, it's a 1.6 Million people city. And the first year when my daughter was taking public transport she was switching lines at Munich central station. In 2015. During those days when there were several thousand refugees here.
You teach the kids about sensible behaviour, about how to travel safely. And then you have to let them take a risk, because locking them up in the apartment is a very dangerous thing, for their mind, for their health.
There IS a certain risk that something bad happens. But I refuse to encumber the kids souls with fear and panic. I definitely believe part of the "obesity epidemic" is parents driving their kids everywhere because "its safer". No it's not. The kids get sick and fat.