here is what California says (verbatim):
§ 21212. Bicycle helmets; wearing and selling helmets in conformance with law; violations and penalties; use of penalty collections
(a) A person under 18 years of age shall not operate a bicycle, or ride upon a bicycle 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 the American National Standards Institute (ANSI Z 90.4 bicycle helmet standard), the Snell Memorial Foundation's Standard for Protective Headgear for Use in Bicycling, or the American Society for Testing Materials (ASTM F-1447 standard). 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.
(b) Any helmet sold or offered for sale for use by operators and passengers of bicycles shall be conspicuously labeled in accordance with the standard described in subdivision (a) which shall constitute the manufacturer's certification that the helmet conforms to the applicable safety standards.
(c) No person shall sell, or offer for sale, for use by an operator or passenger of a bicycle any safety helmet which is not of a type meeting requirements established by this section.
(d) Any charge under this subdivision shall be dismissed when the person charged alleges in court, under oath, that the charge against the person is the first charge against that person under this subdivision, unless it is otherwise established in court that the charge is not the first charge against the person.
(e) Except as provided in subdivision (d), a violation of this section is an infraction punishable by a fine of not more than twenty-five dollars ($25). The parent or legal guardian having control or custody of an unemancipated minor whose conduct violates this section shall be jointly and severally liable with the minor for the amount of the fine imposed pursuant to this subdivision.
(f) Notwithstanding Section 1463 of the Penal Code or any other provision of law, the fines collected for a violation of this section shall be allocated as follows:
(1) Seventy-two and one-half percent of the amount collected shall be deposited in a special account of the county health department, to be used for bicycle safety education and for assisting low-income families in obtaining approved bicycle helmets for children under the age of 18 years, either on a loan or purchase basis. The county may contract for the implementation of this program, which, to the extent practicable, shall be operated in conjunction with the child passenger restraint program pursuant to Section 27360.
(2) Two and one-half percent of the amount collected shall be deposited in the county treasury to be used by the county to administer the program described in paragraph (1).
(3) If the violation occurred within a city, 25 percent of the amount collected shall be transferred to and deposited in the treasury of that city. If the violation occurred in an unincorporated area, this 25 percent shall be deposited and used pursuant to paragraph (1).
Get caught and there seems to be a $25.00 fine.