Miss. Code Ann. § 21-29-31

After the expiration of four (4) years from the effective date of an employer’s operation under this article, any member who has been an employee of the employer, to whom application is made, for twenty (20) years or more, whether continuous or not, provided the last seven (7) years of which service shall have been continuous, may retire upon his written application to the board setting forth at what time, not less than thirty (30) nor more than ninety (90) days subsequent to the filing thereof, he desires to be retired. The board shall grant him the benefits under Section 21-29-33 to which he is entitled because of the service retirement. It shall be mandatory that any member retire after he attains the age of seventy (70) years.