In my state it's illegal for anyone over 12 years old to ride on the sidewalk... though I've also never heard of it being enforced.

Even with no pedestrians to contend with, sidewalks present their own set of risks/dangers - particularly at any driveways or entrances to subdivisions or shopping areas. Those risks can be mitigated though with observant riding and a slower pace, but they're still there. Conversely there are some high traffic / narrow lanes / no shoulder roads which are riskier to be on than the near-abandoned adjacent sidewalk.

I'd say folks ought to make their own evaluation of the safest way to travel through a particular area.