I grew up playing out at the park etc, Raf housing estates both abroad and in the UK, we all played out. When I lived in Cyprus I would go to the beach etc.
Once in the UK my mum got a part time job so I would be on my own and once about 14 I looked after my sister, five years younger. My mum even did night shifts and we would be left alone when my dad was away (often) then in the mornings I would get myself and sister up and ready for school, drop my sister at childminder in morning and then go to school. I was also regularly babysitting for others at that age. And did a paper Round and started working in kitchens in hotels and an oap home. I would get up and cycle to the hotel to do the early morning shift on weekends. I saved a good amount of money for uni with the jobs I did in my teen years. I had to get myself to and from most places on a bike due to parents jobs. My 15 yr old babysits and volunteers part time in a little pc shop in town.
He was given bits of responsibility such as school run, trip to the shop etc on his own so am confident in his abilities. He can get the train to visit relatives and has done so since about 13. So train into London and a connection to make.
Ds2 12 will walk half hour to town to meet friends for cinema or lazer quest etc.
The elder three have mobile phones to contact us if necessary. So actually much easier than it was when I was a kid to stay in contact.
Ds4 is just 7 and will play out in our cul De sac and I would leave him alone whilst I nip to the local shop or with ds1 for longer periods etc. He cycles with me to school. On the next year or so depending I will let him go to the local shop on his own. He can go up the road to a friends. We are lucky the area we live has quiet roads and the little shop and library etc. But we chose it because of the amenities and the cul De sac for the kids to play out etc.
These things all depend on area and most importantly the child themselve.
But parents should imo be teaching children stuff like who to contact in an emergency, basic safety and first aid etc. How to dial 999, safe escape routes etc.