The other example in the parent comment was about fuel economy and trucks. They're just generalized laws about unintended consequences.

I used to walk and ride my bike to school. I was in 4th grade. 9 years old.

You show me a 9-year-old walking alone to school today, and I'll show you a parent who's getting investigated for child neglect. It's maddening.

So that chain of consequences means today's kids are meant to be watched 24/7, and that usually means they're cooped up inside. They're still facing "Stranger Danger" (except through Snap or whatever games they're playing), and now they're also in poorer health.

Nine is a bit young, but I lived down the street from a school and plenty of 10 - 12 year olds walked there by themselves.