Most places have laws for children under 16 or 18. Adults can use their own discretion. I think most commuters in the US wear helmets.
You also have to know that a helmet doesn't stop collisions. It is a last line of defense for some head injuries. They are tested with a drop test from a relatively low height. They do nothing when you are hit by a 3 plus thousand pound vehicle at speed.

I used to commute 12 miles each way in the SF bay area. Always wore a helmet. With a rearview mirror. Lights at night. Had an airhorn. Drivers still paid almost zero attention to me on the road. Defensive riding kept me alive, not a helmet. Helmets saved me when I snapped off pedals twice and shattered them on impact.

I'm now in Munich. Facilities are far superior. Drivers pay attention. I go helmet free on my town bike. I wear a helmet on the fast bikes and group rides. Based on helmet testing I'm probably doing it backwards.

The real answer is better facilities for all road users.