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

Somalia

Under article 141 of Somalia’s Constitution eligible voters have the right to vote in referendums and by secret ballot in elections. The voting age in Somalia is 18 years old. (Constitute Project, “ Somalia’s Constitution of 2012”).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 2012 · Last edited 2022-12-28 · Edit on GitHub