If any member of a participating municipality who is an elective officer is separated from the service of the municipality by which he is employed, except for cause or by resignation, after attaining the age of sixty years and after completing at least twenty years of continuous service but before reaching the voluntary retirement age, he shall be entitled to a retirement allowance upon reaching the voluntary retirement age; provided, at the option of the elective officer, the retirement allowance may commence on the date of such separation and be payable in such an amount as may be determined by the Retirement Commission to be the actuarial equivalent of the retirement allowance that would have been payable except for the election of such option.