I walked 2kms to and from school each day from the age of 4 unsupervised.

That being said, my DS1 is 4 years old right now, and I wouldn't let him walk to the park 200m down the road unsupervised.

I have heard 12 years as being the recommendation for when kids should go out and about alone (I was catching a train for an hour, with a 1km walk there and back from 10yo...) but would like to think that my children will be able to walk to school earlier than that - as long as I was confident that they were safe. I love the idea of a walking bus.