Many jurisdictions explicitly allow younger children to ride bicycles on sidewalks.

In practice though it's almost never enforced.

that's true for many countries in europe too, but we do have a lot more bike paths and drivers are generally more considerate towards cyclists than they are in the u.s.

with my road bike (i.e. no suspension) i usually prefer the road over adjacent bike paths (which is legal in my country), as the roads are usually in better shape.

Yup. Bikes are 'cars' unless being walked. That includes crosswalks.

That depends very much on the location. In my area, for example, bikes are allowed on sidewalks if there is no bike lane.