Maybe the school is saying 10 because they are finding a lot of younger children already walking alone and are concerned. I think it's actually law that children under 10 are supposed to be accompanied to cross the road.

I have a friend, who lives on the same street as our school, who has let her children walk to school for quite a while. The oldest child was 8, walking her 6 year old brother. I live one street back from the school, my oldest is almost 9 and I won't be letting him walk without an adult for a long while lol.