The right to vote at the age of 18 – 1971
“Section 1: The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age.
Section 2: The Congress shall have power to enforce this article by appropriate legislation.”
This came about due to arguments that if you are old enough to be drafted you are old enough to vote.
Side note: Fearing confusion with the upcoming 1972 election Congress quickly proposed and passed this amendment permanently lowering the voting age to 18 and the states ratified this within four months.
See: Previous Amendment (Twenty-Fifth Amendment) – Next Amendment (Twenty-Seventh Amendment)