Every person 18 years of age or more who has been a citizen of the United States for three months and who has resided in the precinct for 30 days next preceding an election shall be entitled to vote in that precinct. The place of voting by one otherwise qualified who has changed his residence within 30 days preceding the election shall be prescribed by law. The following persons shall not be entitled or permitted to vote at any election in this state: A person not meeting the above requirements; a person who has been convicted of treason or felony, unless restored to civil rights; a person under guardianship, or a person who is insane or not mentally competent.