What is the oldest written source in this country that mentions this right?
Marshall Islands
According to Section 3, elections of the members of Nitijela shall be conducted via a secret ballot system based on universal suffrage of those who have attained the age of 18 years or greater unless they are certified insane or are currently serving time for a felony. (Constitute Project, “Marshall Islands’ Constitution of 1979 with Amendments through 1995”).