Appropriate age for leaving child alone

[NeeChee]

DSS is 10, 11 in 2 months.
Me and DP were discussing his childcare plans for the summer holidays, and he mentioned that his mum is thinking of leaving DS alone at home whilst she is at work.
He is quite mature, and I think he would likely be fine, but me and DP have never left him home alone at all, never mind for all day. I personally think it'd be a big jump to go from nothing to a full 8/9/10 hour day.
What is everyone else's experience?
I think it would be better to phase him into it, maybe an hour or a couple of hours at first.
I think it would be good to encourge him to be more independent. He'll be moving up schools next September (2019), and we're thinking of letting him make his own way to and from school, and he'd probably be alone for an hour or two before one of us arrived home from work. At the moment he goes to breakfast/after school club and is then picked up, usually by DP.

I've started leaving my 10 year old home alone for up to an hour, which feels like enough to me at the moment. I think a whole day is too much at that age, especially if he's not used to being left alone.

I started leaving my 8yo for a few minutes and now 10 he is ok to be left for about 2 hours. After that he says he gets lonely and feels down so he’s clearly reached his limit for now. Have you asked your dss how he feels about it.

Leaving a child who isn’t used to it, alone for whole days is not on. He may get distressed, lonely or frightened. Does he know how to cope with a fire or a power cut?

As a single mum I understand the necessity but there are holiday clubs available.

My DS were both just 11 when we started leaving them for a couple of hours, we live on an isolated country road with only one neighbour and both work 30-40 mins away. No family in the county! Even now DS2 is just 13 I'll leave him for half a day max. People's circumstances and Dc are so different.

I am an 12yr old boy and at 11 I only stayed alone for 30 mins at a time, and I did only go alone to a nearby minimarket. Now I am quite independent, I sometimes stay alone up to several hours (with a microwave meal I can stay at home from morning until evening), and I return home from school and go and return from my activities on my own (I have a key of my home and I fully master the public transport system). But it was not a quick transition, and quick transitions from no time alone to a day are difficult for child and parent alike.

It seems more likely now that he will end up at the local secondary school, which is walking distance. Which would mean he could get himself to school and back, and would only be alone for maybe an hour, an hour and a half in the evenings.
I'm just slightly concerned that he will forget to lock the front door as he seems to be obsessed by his phone at the moment. And I can see him walking out of the door while staring at it and just forget.
Also there's the responsibility of feeding the dog and letting him out (whoever gets in/up first does this to avoid any accidents). I've known him get up, ignore the dog and go straight on his PlayStation before.