The East Bay Regional Park District's Volunteer Trail Safety Patrol encourages you to follow its members' example: wear a helmet when riding on wheels or on horseback on District trails and roadways. Regardless of your age, your helmet reduces the likelihood that you will suffer serious head injury. It can save your life.
California state law requires that children (persons under the age of 18) wear a helmet when riding almost anything with wheels on public trails, streets, bikeways, and sidewalks everywhere in the state, including within the District. Helmets are required for the following activities:
The helmet must be properly fitted and fastened and must meet established safety standards.
For those interested, here is actual text from the California Vehicle Code as it appeared in March 2014:
Section 231. A bicycle is a device upon which any person may ride, propelled exclusively by human power through a belt, chain, or gears, and having one or more wheels.
Section 21212. (a) A person under 18 years of age shall not operate a bicycle, a nonmotorized scooter, or a skateboard, nor shall they wear in-line or roller skates, nor ride upon a bicycle, a nonmotorized scooter, or a skateboard as a passenger, upon a street, bikeway, as defined in Section 890.4 of the Streets and Highways Code, or any other public bicycle path or trail unless that person is wearing a properly fitted and fastened bicycle helmet that meets the standards of either the American Society for Testing and Materials (ASTM) or the United States Consumer Product Safety Commission (CPSC), or standards subsequently established by those entities. This requirement also applies to a person who rides upon a bicycle while in a restraining seat that is attached to the bicycle or in a trailer towed by the bicycle.