Mo. Rev. Stat. § 56.010

At the general election to be held in this state in the year A.D. 1982, and every four years thereafter, there shall be elected in each county of this state a prosecuting attorney, who shall be a person learned in the law, duly licensed to practice as an attorney at law in this state, and enrolled as such, at least twenty-one years of age, and who has been a bona fide resident of the county in which he seeks election for twelve months next preceding the date of the general election at which he is a candidate for such office and shall hold his office for four years, and until his successor is elected, commissioned and qualified.