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

Croatia

Under Title II, Article 45, all birthright citizens 18 years or older, have access to universal, and equal suffrage through secret and direct ballots to determine the Croatian Parliament, President of the Republic of Croatia, and the European Parliament. (Constitute Project, “Croatia’s Constitution of 1991 with revisions through 2013).

Voting Rights and Suffrage · Country sources (all rights) · Years mentioned: 1991, 2013 · Last edited 2022-12-28 · Edit on GitHub