It's really not so much about their age as it is how responsible/mature they are and under what circumstance.
When I was growing up me and my younger brothers were raised by a single mother who worked full-time as a Registered Nurse. Although we did not have a name for it back then we were essentially "latch key children". We walked to school alone (approximately 5-6 blocks) in kindergarten. I must have been around age 7 when we stopped having a baby sitter waiting for us at the house. We were given strict orders to stay inside, lock the door, and not let anyone in the house...etc If we didn't we got whipped. Today it would be considered "child abuse".
A couple of years later she'd often work the swing and graveyard shifts which meant we learned to heat up left overs or cook our own dinners and sometimes breakfasts. Periodically she'd call home to check in on us.
Today you see kids riding tricycles with helmets and knee pads. Parents would be afraid to let a kindergarten child walk to and from school alone. They'd also risk having the county take their children away if it was known they were left in a house overnight alone. Times were different and expectations of children were also different. There was no Internet or numerous other ways to engage with strangers once you got home safely.