California Code, Government Code - GOV § 21118

(a) A local member shall be partially retired for service upon his or her written application to the board if he or she has elected to participate in partial service retirement pursuant to Sections 21110 through 21115, provided he or she has attained the applicable normal retirement age as prescribed by regulations of the board.

(b) This section shall not apply to a contracting agency or its employees until the contracting agency elects to be subject to it by amendment to its contract made in a manner prescribed for approval of contracts or in the case of a new contract, by express provision of the contract. The operative date of this section with respect to a local member shall be the effective date of the amendment to his or her employer's contract electing to be subject to this section.