At what age is it OK to leave a child on their own?

Good point niggly

I looked after my 3 nephews aged 1,2 and 5 for the weekend when i was only 9 whilst my sister went house hunting in another town. Twice I also traveled on the night train from S.W Scotland to London Euston, across London then on to Kent on my own aged 11, the train was full of very loud Northern Irish men boozing away but i never felt unsafe, my father put me on the train and told the guys to look after me and they did.

Hotel porters are probably not DBS checked!
Kids can have a bad dream and wake up feeling disorientated and scared. Particularly in a strange place.
I feel sorry for the young of the Goves. Enough to give anyone a nightmare.
I would not leave child in the house below 13/14 because homes are where most accidents take place. One DS when a teenager managed to break his leg when in the house on his own by knocking over some plaster boards the builders had left.
There is loads of scope for injuring oneself in the kitchen alone, if they decided to make a snack or a hot drink.
I did once allow DS1 to travel from Paddington to Swansea on his own because he was desperate to see his DGM.
Sat him next to an older woman who was travelling all the way and he was fine. But kids don't have a lot of scope to be not fine on intercity trains where they are getting on at the terminal off at the last stop. And lots of kids do unaccompanied flights with the staff looking after them at each end.

Most of us here will be of an age to remember that Bobby & Ethel Kennedy left their 13 year old son alone in a hotel room in June 1968 (indeed, many of us would have been a similar age, or had a sibling that age, as I did)

We didn't have all-night TV here in the UK, but they did in the USA. He had woken, put on the TV and saw the news of his father. A reporter, wandering the corridor, trying to get quotes heard the child crying, knocked on the door, and was the first to attempt to comfort him.
That stuck with me. I expect Bobby & Ethel expected to be back relatively early.

As for the original question, it does depend on so many factors, and for me, the extra 4 hours has a great deal of significance (and makes me wonder why they stayed on so long?)

I was a latch-key kid too. At 10 I would get home from school, start preparing vegetables and set the table for the evening meal and, in the winter, get the fire started.

I was left babysitting from about 8 years of age. The first time we went out and left ours alone, the eldest was 12, and we were so worried, we were back home within an hour! They were all disappointed to see us back.