In the United States, most states do not define the need to wear a helmet when riding or operating an electric bike. However, some states do require helmets. These requirements will vary depending on age and bike class. If you need to buy a new bike helmet (and I always recommend them regardless of any regulations), then now is the best time to do it.
I went through and compiled a list of the current helmet laws for each state which you can see below:
All passengers & operators
Operators of Class 3 under 21
Operators & passengers of class 3 under 18
Operators & passengers of class 3
No definition by state
Wear a Helmet Anyways
Regardless of helmet laws, I highly, HIGHLY suggest to always wear a helmet while biking. Mountain biking, or really any biking, can be dangerous and this is such a cheap way to radically low the chances of serious injury. Take a look at my article of the essential gear to bring while biking to ensure you are ready for anything that comes up. When you’re miles away from help, you need to be prepared.
Outside of that, make sure you are prepared and then have some fun. I would suggest starting out on mild trails and working your way up as your skills and bravery levels increase.