Any district attorney of this state who has served continuously for 15 years as district attorney and who is not less than 70 years of age may elect to become a supernumerary district attorney of the state by filing, while in service, a written declaration to that effect with the Governor, who shall, if the age of the declarant is not less than 70 and his continuous service as district attorney is not less than 15 years, thereupon issue to such declarant a commission as supernumerary district attorney of the State of Alabama.