I was a latch key kid at 8 years old walking home from primary school by myself with my sister who was 2 school years below me, so year 2. That was crossing a main road with a lollipop lady and then residential streets.
My parents both worked full time.
School was a 5 minute walk away so I would have been home at 3.35/3.40 and my parents made it back somewhere around 5.30. I hated it, I hated the responsibility and I was terrified, especially in winter. I was also expected to make my own packed lunch. Looking back I would say it was neglectful, why wouldn't she make my flipping lunch?
My own children, ds1 is almost 12 and in year 7 so walks to school and lets himself into the house, I make it back 10 minutes later as I have to collect ds2 in the opposite direction. I do leave ds1 for an hour at a time but no more because the occasion hasn't arisen. But I would.
I don't work, Dh can work from home so we can always have someone here if needed.
But with mobile phones and the ability to contact someone immediately for help or advice or just to talk if they were a bit afraid, I cannot see why we just don't equip the children with these skills.
You have to talk to them along the line of "what if...." and then talk them through what to do. I have 2 sons, I can't take them into the toilets with me when out and haven't done for a long time. So we talked through different scenarios and what to do. The same for leaving them alone, you don't answer the door, what food they are allowed to have in your absence, etc
It should be done on a case by case basis, one 9 year old can be far more mature than some 14 year olds.