I live somewhere it's still usual for children to 'play out'... But it's less usual than it was 20 years ago, when I first moved here. I have been in the same house for all that time, so I can see clear changes:
20 years ago, our street was full of children - toddlers to teens. Children walked alone or with friends to the village school from the age of about 6. Pairs or groups of children all aged 6-7-8, with one or two younger, were very common out and about all round the village.
10 years ago, a lot of children played out on the street, but mostly from the age of 3 or 4. Children that age were often in the care of slightly older children. All children walked to school alone from the age of 7, when they started juniors, and any parent who didn't allow this would have been thought seriously odd! At the same age, children would go to play areas round the corner or other places close but out of sight of parents. From the age of 8 or 9, kids were going up onto nearby moors without adults - not necessarily with permission, but because they were 'playing out' and their parents didn't know exactly where they were.
5 years ago, the street had got quieter, but other than that, habits hadn't changed much.
These days, there are only a handful of children on the street. It's true there are fewer young children living here (most of the families have stayed put, so their children have grown up) but the rest of the village is quieter too. Children play in the park and play areas more than on the streets, and because these are further away from most people's homes, the children tend to be supervised by adults more often. It's definitely my impression that children play out much less often, generally. Kids still walk to school alone from the age of about 7, but more of them are accompanied or driven there. Children play out entirely unsupervised from the age of about 10-11-12. Generally, I'd say children stop being supervised the year before or after they start high school, rather than the year they start juniors, as it was 10 years ago.