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

Djibouti

Under Title I, Article V, all Djiboutian nationals of majority have a right to Suffrage regardless of gender. (Constitute Project, “Djibouti’s Constitution of 1992 with revisions through 2010).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 1992, 2010 · Last edited 2022-12-28 · Edit on GitHub