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

Tuvalu

Article 87 of the Tuvalu Constitution states that the members of Parliament are voted into office by voting age (18 years old) adults based on the principles of universal suffrage. (Constitute Project, “Tuvalu’s Constitution of 1986 with Amendments through 2010”).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 1986, 2010 · Last edited 2022-12-28 · Edit on GitHub