Legally it's definitely 13yrs before they can be left alone, but parental judgement should also be used, as some children may not be mature enough at that age. (No offence intended towards any child or parent, just a point of view)
Actually there is NO legal age limit - it is purely at the discretion of the parents.
However .... social services CAN and WILL intervene if they think that the child was left in a situation that is dangerous to them.
Times have certainly changed when i was 9 i used to walk home let my self in and cook the dinner!! now i dont even like the idea of a child walking to or from school on their own!! the world is too scarey now!
Sorry havent answered question, but i think you should judge by how mature the child is.
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My sister used to let her 2-year old play in the garden when she was in the house, whereas I would not do that. I can barely allow my children (3 and 5) to be in a different room from me in the house because they either wreck the place or they fight!
I was 11 and my brother was 10 when we were allowed to come home from school and stay there for 2 hours before Mum and Dad got back from work. To be honest we were probably safer there than with some of the so called childminders we had whose own children often used to get us to do dodgy things e.g. I remember one game of throwing a glass bottle around unsupervised. I think it probably depends on how mature the kids are.