What is the oldest written source in this country that mentions this right?

Costa Rica

According to Title VIII, Chapter II, all birthright citizens 18 years or older and naturalized citizens, 12 months or greater after naturalization, have the right to suffrage facilities. (Constitute Project, “Costa Rica’s Constitution of 1949 with revisions through 2020).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 1949, 2020 · Last edited 2022-12-28 · Edit on GitHub