California Code, Government Code - GOV § 31552

All existing officers and employees of the county become members of the association on the day the retirement system becomes operative, and thereafter each person entering the county employ becomes a member on the first day of the calendar month after his entrance into the service, unless otherwise provided by regulations adopted by the board.  Such regulations may provide for waiver of membership by the prospective employee in the case of newly hired employees who have attained the age of 60.  In all cases where there is such a waiver, said employee upon attaining the age of 70 shall thereafter be employed from year to year at the discretion of the county.