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

Turkmenistan

Under Article 119, citizens of Turkmenistan who are at least 18 years old can vote for the President of Turkmenistan, the deputies of the Mejlis, and members of the People’s Council. (Constitute Project, “Turkmenistan’s Constitution of 2008 with Amendments through 2016”).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 2008, 2016 · Last edited 2022-12-28 · Edit on GitHub