Each employee of a district shall be eligible to participate in the plan adopted if he has been continuously employed by the district for a period of not less than twenty-four months prior to the effective date of the plan adopted by a district, or for such longer period as may be provided in the plan. The plan may provide that an employee who shall have attained a specified age or who has been continuously employed by the district a specified number of years will not be eligible. Each other employee shall become eligible to participate on the first day of the month coinciding with or immediately following the completion of not less than twenty-four months continuous service with the district or such longer period as may be provided in the plan adopted. Each eligible employee shall become a participant as of his first eligibility date provided he makes a written application to the district on forms furnished by it within one month after his first date of eligibility. Each other eligible employee may become a participant on the first day of the month following the month in which he makes written application therefor, in which case he will be ineligible for prior service retirement benefits which may be provided by the plan.