My 15 yr old babysits for my younger four, I put the little four to bed and once they are asleep I go out. Has done so for over a year. To begin with it was just whilst I went for a quick run, so 30 mins. Now I will go out to pta meetings or a meal etc. He also babysits for friends kids, he has built up a good little client base and been able to save enough money to build a pc! He is Mr sensible and very used to kids having four younger siblings.
I won't leave him with them in the day, well not all of them. Will leave him the 12 and 10 yr old or the 7 yr old.
My 10 and 12 yr old can both be left home alone for a period of time. The 10 yr old cycles two miles to school on his own or home alone if I am taking the younger two to clubs. The elder two catch a bus six miles to and from school and in a year the 10 yr old may do this as wrll depending on which high school he goes to.
They have been allowed age appropriate independence, going to two mins to a local shop or five mins to the library, playing out in our cul De sac. Building up gradually. The big two can go to town to the cinema etc.
As a parent it's my job to help them on this journey to independence. There is no magical age at which it's Ok, it depends on the child. In many ways my 10yrs old is more sensible than my 12 yr old. Bur he has asc so it's even more important we teach him sensibly.
My dp works in child protection so is fairly cautious but he is happy with the decisions we have made together in this respect.
You couldn't make a blanket law and it's not necessary. The current law allows judgement to be made yet allows for intervention if necessary as it should be.