California Code, Government Code - GOV § 31770.5

(a) The implementing ordinance shall establish the eligibility requirements for participation in the program, subject to this section and the collective bargaining agreement. The ordinance shall specify the minimum age and the minimum and maximum, if any, years of service credit required to be eligible to participate in the program, which minimum and maximum, if any, may not be less than the minimum age and service credit requirements for service retirement.

(b) Members shall be eligible to elect to participate in the program at any time after the attainment of the minimum age and years of credited service in the system specified in the implementing ordinance. Members who satisfy the eligibility requirements on the implementation date of the program, as set forth in the implementing ordinance, shall be eligible to elect to participate in the program as of the operative date of the program.

(c) Prior service purchased pursuant to this chapter and service performed by the member under another public retirement system shall be included for purposes of determining eligibility for the program to the extent provided in Section 31836.

(d) Members who have left county or district service and who have elected deferred retirement pursuant to Article 9 (commencing with Section 31700) will not be eligible to participate in the forward DROP provisions unless they return to county service during the operative period of the program.