Mo. Rev. Stat. § 70.645
Any member in service may retire with an allowance provided for in section 70.655 upon his written application to the board setting forth at what time not less than thirty days nor more than ninety days subsequent to the execution and filing of his application he desires to be retired; except that, at the time specified for his retirement the member must have attained his minimum service retirement age, or if an election has been made in accordance with section 70.646 to provide for alternate eligibility, have years of attained age and years of credited service in force which total eighty or more, and must have five or more years of credited service in force, and notwithstanding that during the period of notification he may have separated from service. He shall have the right to elect an option provided for in section 70.660.
§ 70.645, RSMo