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

Eswatini

Under Chapter VII, The Legislature, Section 88, Qualifications as a Voter, a person is qualified to vote if they are of the age of 18 and a citizen or ordinarily resident in Swaziland (Constitute Project, “Eswatini Constitution of 2005”).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 2005 · Last edited 2022-12-28 · Edit on GitHub