It''s a very difficult one. Interestingly I don''t think there is a legal age on it if the child is being left on own. It is different if they are being left responsible for a younger brother/sister.
I think you have to think about the circumstances/childs maturity.
Personally I don''t feel that it is acceptable and I was most distressed when my ex allowed our 10 and 8 year old to walk unaccompanied to a friends house. I mentioned it at the end of a Directions Hearing to the Judge and the Judge also agreed.