It's state by state here, & a bit of a mix.
Off the top of my head.
WA is mandatory for all. This is the most common.
MN, last I checked, is mandatory for minors (less than 18). Adults can assume their own risk.
FL is not mandatory for adults, not sure about minors. One of the folks who pushed removing the helmet law died of head injuries while riding without a helmet as I recall.
I really try not judge those who don't use one, but personally I did use my helmet all the time while in FL.
Helmets are a legal requirement in the EU, which is good for anybody who gets hit - it's amazing to us that there's no requirement for it in other parts of the world!