California Code, Government Code - GOV § 21196

The board may reinstate a person from retirement upon (a) his or her application to the board for reinstatement and (b) the determination of the board that his or her age at the date of application for reinstatement is at least six months less than the age of compulsory retirement for service applicable to members of the class or category in which it is proposed to employ him or her. The provisions of clause (b) of this section shall apply only to patrol, state peace officer/firefighters, and safety members. The effective date of reinstatement for purposes of this article shall be the first day of compensated employment following approval of reinstatement.