When I was in year 4-6, yes I did walk to school, with my brother who is 2 years younger. We lived in a country village though, where all the kids who lived in walking distance walked. It seemed like a normal and safe thing to do. Once we hit high school we had to catch a bus as it was about 10km away.
DS is only 15 months old, so it's a while before he'll be at school. I can't imagine letting him walk alone to school until he is in late primary school maybe? Depends on how responsible and street smart he is I guess.
Truth is, my mum will most likely care for him before/after school, so it probably won't be an issue.