The world has changed. At 6 I was walking a mile to school, or getting a London Transport bus. At 9 I was travelling by underground and overground trains from Wimbledon to Whitton by one of two ways, one involving changing at Clapham Junction. I haven't a clue at what age I would now let a child walk to school. I live very rurally, and it would involve walking down a busy main road and country lanes for about 2 or 3 miles. I let my son cycle to school when he was 9 or 10, but that was back in the late 80s. My main reason for worrying would be about the huge increae in traffic.
I have worked in schools and their is a vast difference in the maturity of kids, some are still like babies when they come to school, others are quite capable of doing a lot more. Children that are coddled are a nightmare!