Depended where we lived (DF was in the army, so we moved constantly). At 7, with my 5 year old sister, we had freedom to go wherever we wanted, unsupervised, living in a small rural village in Yorkshire.
Even living in Hong Kong, aged 10 and 8, my sister and I caught a bus, ferry and another bus to get to school on our own.
For DGC, living in a town close to a busy arterial road, they started walking to school alone at 9, not far but with the main road to cross and 3 busy side roads. At 10 they started to go elsewhere by themselves preparatory for 'big' school. DGS is currently meeting with a friend once a week and walking a mile to the local shops to do some shopping, preparatory for a 2 mile journey to secondary school this autumn.
I think the biggest problem nowadays is traffic. There was hardly any when I was small, now DGCs road is lined with parked cars with traffic constantly passing up and down and a very busy road at the bottom, as I said.