Standards have definitely changed over the past two generations. When I was young in the sixties we were allowed infinite freedom from an early age and made our own way to and from school. We were warned to keep away from 'strange men' but not monitored much beyond that. My mother worked and, in the school holidays, I roamed far and wide on my bike exploring and learning about the world. My DH says the same about his youth and, at one point, he'd get on buses and go as far as he could. As well as this he used to cycle up a long, isolated, private concrete road into the countryside without anyone knowing where he was. My old Head of Department told tales of running across a canal pipe where he could easily have fallen and drowned.
However, the country seemed (and probably was ) safer then and no harm came to any of us. We became independent and resilient and learned far more about the world than our own children did.
Today it is very different. We are aware of what the 'strange men' get up to and the internet has given them confidence and power and has encouraged them to go out and seek victims. My own children were NEVER left on their own in the house or outside it. When my daughter was 14 she had a hissy fit and ran off. I think the ten minutes she was missing were the longest of my life. Nevertheless, from the children being the age of about ten we used to encourage growing independence and allowed them to visit the green to play or to walk to friends or do errands at the shop over the road. Mobile phones helped considerably for checking up and checking in purposes. Children need to learn how to be independent, responsible and sensible.
As to the degree of independence allowed and the timing of it, it is up to the maturity and sense of the individual child. Mine were sassy and outspoken and we used to joke that if anyone kidnapped them they'd be back on our doorsteps in half an hour with a ransom offered to take them back.
But it's not a joking subject, really and I was a hypocrite about mine as, while I think a degree of discretion should be allowed to parents in leaving their children, I never, ever did.