It was when I was a kid.

I trusted my then-3 year old to be 250' away at the opposite end of a neighborhood park that abuts my house. Usually within eye sight but not always.

Once he reached 5 and showed they understood the boundaries and requirements, to go out there by himself.

And once he reached 6, he was trusted to look after his 4 year old brother in this space.

And these restrictions seem very strict compared to how I grew up and especially compared to how those older than me grew up. Or in some other places in the world, where e.g. in Japan it's not rare for 6 year olds to ride the subway and do errands and downright common for 7-8 year olds.