I agree that 8 is too young...as for what age is ok I think it depends on the child.
DS2 is 7 & I can't imagine leaving him alone at 8. We do leave him with his almost 12 yr old brother though if we are going to the supermarket (or his 14yo sister when she's around). DS1 is 12 in a few weeks & we leave him on his own. He likes it makes him feel grown up
There are other factors...we live in a gated complex & it's full of adults who know us & our kids so if they really needed help it wouldn't be far away.

I like Mrsmac's idea, if you can find some literature stating that 8 is too young...

Failing that it might be an idea to make sure that your brother has a backup plan... can he call someone if he needs, how far away would help be, are there neighbours nearby that he knows, etc.

ETA Mantaray isn't it funny how times have changed! (now I sound old lol) my stepmum used to leave 8yo me with my 4yo sister while she went power walking. I wouldn't dream of it myself...