At What Age Is It Appropriate For A Child Walk Home Alone?

I spotted a story in the headlines yesterday regarding a couple in Maryland who practice "Free Range" parenting and have landed themselves in Hot water with child protective services.

The latest incident occurred last weekend when the parent allowed the children to play alone at a park and then walk home. FYI the park is about a mile from the families home.

This had my Husband and I in a lively conversation last night. We both recall walking home from school when we were in kindergarten or about age 5 and 6. I realize we live in different times but what age would you feel comfortable letting you kids walk home from school or play at a park alone?