The prior service retirement benefit shall be a straight life annuity, payable monthly with the first payment made as of the annuity start date, in an amount determined in accordance with the State Employees Retirement Act, except that the payments may be made less often than monthly if the monthly payment would be less than fifteen dollars. At the option of the member, the first payment may be deferred to commence at any time, except that no benefit shall be deferred later than the required beginning date. Such deferred benefit shall be the actuarial equivalent, based on factors designated by the board, of the prior service benefit. In the event of retirement before age sixty-five under section 84-1317, the amount of the prior service annuity shall be reduced in accordance with the principles of actuarial equivalence based on factors designated by the board. Any member of the retirement system who ceases to be an employee before becoming eligible for retirement under section 84-1317, who has accrued a prior service retirement benefit as defined in the act, and who has been continuously employed by the state for ten or more years immediately prior to termination shall receive the prior service retirement benefit determined in accordance with the act upon attaining age sixty-five. At the option of the terminating member, such annuity may commence as of the first of the month at any time after such member attains the age of fifty-five or may be deferred, except that no benefit shall be deferred later than the required beginning date. Such election by the terminating member may be made at any time prior to the commencement of the annuity payments. Any terminating employee who forfeits a vested future service retirement benefit by withdrawing his or her employee account shall also forfeit any vested prior service retirement benefit to which he or she would otherwise be entitled.