Idaho's current motorcycle laws:

49-665. RIDING ON MOTORCYCLES. A person operating a motorcycle shall ride only upon the permanent and regular seat attached to it, and the operator shall not carry any other person nor shall any other person ride on a motorcycle unless the motorcycle is designed to carry more than one (1) person, in which event a passenger may ride upon the permanent and regular seat if designed for two (2) persons, or upon another seat firmly attached to the rear or side of the vehicle.

49-666. MOTORCYCLE SAFETY HELMETS -- REQUIREMENTS AND STANDARDS. No person under eighteen (18) years of age shall ride upon a motorcycle as operator or passenger, upon any highway, unless at all times when so operating or riding upon the vehicle he is wearing, as part of his motorcycle equipment, a protective safety helmet of a type and quality equal to or better than the standards established for helmets by the director.