Any member of the police department, including officers and police matrons, who shall have served twenty years or more in the department and shall have reached the age of sixty years, or who, while a member of such department shall suffer permanent mental or physical disability so that the person is unable to discharge the person’s duties, shall be entitled to be retired. Upon retirement, the person may be paid out of the pension fund of the department a monthly pension equal to sixty percent of the average monthly salary received during the highest paid consecutive thirty-six-month period of the person’s employment in the department. If any member shall have served twenty years in the department but shall not have reached the age of sixty years, the person shall be entitled to retirement, but no pension shall be paid while the person lives until the person reaches the age of sixty years, except as provided in section 40-45-11. Any member who has served one hundred eighty months but less than two hundred forty months, has contributed voluntarily to the police pension fund, and who voluntarily and while in good standing as a member has left employment of such city, shall be entitled to elect retirement instead of refund at a pension equal to the proportion of a full pension which the total number of months employed by the city bears to two hundred forty months, but no pension shall be paid while the person lives until the person reaches the age of sixty years. Notwithstanding the provisions of this section, upon the approval of the governing body of the municipality, and at the discretion of the board of trustees if it shall find that the actuarial soundness of the fund would not be materially impaired, any member otherwise qualifying in accordance with the provisions of this section may retire if the person has reached the age of fifty-five years.