A person shall not be eligible for a county office, whether elective or appointive, nor shall a certificate of election or commission issue to any person, unless he is, at the time of his election or appointment, eighteen years of age or over, a resident of the state, an elector of the county or precinct in which the duties of the office are to be exercised and able to read and write the English language. The board of supervisors shall be the sole judge of such qualifications, subject to review by certiorari in the superior court.