No person, who is eligible for retirement under the laws of this state pertaining to the System, shall serve in any capacity as a member of any fire department of any municipality of this state after having attained the age of sixty-five (65) years, provided, however, no person shall be required to retire because of the provisions of this section until such person shall have completed twenty (20) years service.
Okla. Stat. tit. 11, § 49-134