Cal. Government Code § 31558.5

All members employed by a county having a population in excess of 2,000,000 who are employed on and prior to January 1, 1958, whose duties are described in Section 31470.6 and who prior to that date file with the board a written election to become safety members, shall become safety members on January 1, 1958. All members employed by a county having a population of 2,000,000 or less and in excess of 500,000, who are employed on and prior to January 1, 1960, whose duties are described in Section 31470.6 and who prior to that date file with the board a written election to become safety members, shall become safety members on January 1, 1960. After January 1, 1958, each person not over 35 years of age when employed by a county having a population in excess of 2,000,000 in a position the principal duties of which are described in Section 31470.6, shall become a safety member on the first day of the calendar month following his entrance into the service. After January 1, 1960, each person not over 35 years of age when employed by a county having a population of 2,000,000 or less and in excess of 500,000 in a position the principal duties of which are described in Section 31470.6, shall become a safety member on the first day of the calendar month following his entrance into the service.