169.320.  Member may retire, when. — The board of trustees shall allow retirement of a member on a service retirement allowance under the following circumstances:  Any vested member may retire on an annual service retirement allowance upon written application to the board of trustees setting forth at what future time the person desires to be retired; provided, that the member at the time so specified for retirement shall have attained the minimum normal retirement age, notwithstanding that during such period of notification or at any other time the person may have separated from service.  Any member having thirty years of creditable service regardless of age may voluntarily retire upon giving advance written notice to the board of trustees and shall thereafter be entitled to receive monthly payments which are the actuarial equivalent of the monthly payments to which the person would have been entitled had the person's retirement been at the minimum normal retirement age.  Any member who desires to retire under the provisions of section 169.563 shall give the board of trustees at least sixty days' written notice of his or her desire to do so.