Each culture is a little different. What are you going to do with your kids when the parents are working is a problem? At what time do you trust them to stay home by themself?
And from the age of 9 there was no after-school options at least in my school.
Based on their level of maturity and actual evident danger in the immediate surrounding area, 8 on the low end, 12 or maaybe 13 on the high end, anything higher and I'd say something's gone wrong, perhaps at the societal level.
As a GenXer, ~12 (?):
I'm pretty sure my parents stopped hiring baby sitters once my sister turned 12.
But even aside from that, I was allowed to play outside unsupervised at 7 years old. Though we did live on a Navy base at the time, so I wonder if that played a factor.
15 is bonkers. There should be no need to keep your teenager on-leash for that long.
It's also unlikely that she will be convicted since VA passed a law in 2023 that makes it explicit that parents are allowed to let their children go places alone.