I am 34 - my Mum was a single working parent, and she didn't leave me home overnight until I was 15....with my 17 year old (full time employed, driving etc) brother.
Anyhoo - back to the legality parts. People who are saying there is no legal limit are totally right. It is down to the parent to decide if they feel their child is responsible enough to be left.
BUT (and it's a BIG one)
Should a parent decide to leave a child and something (such as an accident, illness, incident) bad happens the parent WILL be held responsible - because they have responsibility for their child and are therefore liable should anything happen to them whether they are there or not, unless that responsibility is transferred to another appropriate ADULT. End of story, shit bust.
It's one of those situations that is fine until it proves itself not to be - if you think your child is responsible enough, and nothing happens then you were right - they are responsible enough and everything is hunkydory. If you think your child is responsible and something happens, even if it's totally unpredicatable and an accident then you were wrong - and you're basically fucked. Because once something goes wrong and you have CHOSEN to leave your children you're heading down the failure to protect from harm/neglect path - because you have parental responsibility to protect them from harm - and you aren't doing that if you left them alone and they were harmed in your absence.
Where I am if something happened to a child under 16 and the parent was away for the night Social Services WOULD get involved. Absolutely guaranteed. In addition if a child had an accident and tipped up at the hospital in the middle of the night and the parent was away SS would be the first call the hospital would make, followed by the police. That's the procedure.
If a neighbour called SS and said two children this age had been left home alone overnight SS would undertake a check as a starting point - and if they felt, on assessment, that your children weren't responsible enough, then again, you could be in the shit.
So yes, you can leave them at that age, but you are not absolved of your responsibility and if anything happens whilst you are not there then you will be in the shit for leaving them because as a parent you are responsible for them and therefore liable for anything that happens to them - unless you leave them in the care of another adult. And even then you have responsibility for checking that adult is safe, and if you choose badly you can be held responsible for what they may do to your kids. That's parental responsibility - which in law is non-transferable even to another caregiver.
People may think that is cotton wool - but that's just how it is.
Would I do it at that age? Hell no.