This is really not representative for other Western countries. Where I'm from, I would say that 75% of 6 year olds walk/bike to school alone, and 100% of older kids do.
> In physical space, Western children are almost comically sheltered.
I think the author should stick with "kids in the USA" when he means that.
When I was growing up in the 80's in the US, I walked to school alone; but there was institutional support for that. There were adults paid to help kids cross major roads, and there were older children ("safetys") who wore an orange sash, trained and assigned to help younger kids walk to school.
I don't see that same infrastructure here in England. I'd be happy to let my 5-year-old walk to a local school if it were present.
EDIT: To be clear, I said "I don't see...", not "There are no...". It's possible I just haven't noticed, or that it's a quirk of my locality. And, my son doesn't go to a local school, so it's a bit moot; he can cycle to his school when he's older.
I'd say even then, five is a little low by UK standards. I think I started walking solo to school when I was about 7 or so. By 11 I was walking a mile to a train station and catching the train 15 miles to another town (nearest Catholic secondary/high school). 5 years old is not far off 7 or 11, but it's a big chasm - be ready for it to go by in a flash! :)