Senators shall be at least twenty-five years of age, and representatives twenty-one years of age at the time of their election. They shall have been citizens and residents of this state for three years and residents of their respective counties or districts for one year next before their election, if such county or district shall have been so long established; but if not, then of the county or district from which the same shall have been taken; and they shall reside in their respective counties or districts during their terms of office.