I had heard 12yo as being the ok age for walking to school.
In our area, as it is a small town and the school only has 160 students, there are many students who walk or ride unsupervised under the age of 10.
There is a walking school bus, but it is only 3 days per week due to lack of volunteers.
The township is mainly just one long road, so most students walk it. Many parents still drop off, plus many locals are around. As a community most people there know each other, so it would be pretty hard to a kid to be taken. If I saw a car pull over near a group of kids, I would take note.

As mentioned before, traffic itself is the main concern. Some of the students are pretty reckless on their bikes, plus many really don't take any notice of driveways.