Of course our activity levels change and in some cases go down as we age, but I'd like to submit that is a given, and that car-only infrastructure is an additional barrier on top of those natural tendencies.

It's simply much easier to walk to a coffee shop, or park, or wherever for those who have maintained their mobility (probably in part by living in a walking-centric environment) than it is to hop in a car, sit in traffic, for small things. It's less of a barrier.