Amendment 26 to the U.S. Constitution
Short summary -- Right to vote for 18, 19 and 20 year olds
Actual text of Amendment 26:
Section 1. The right of citizens of the United States, who are 18 years of age or older, to vote, shall not be denied or abridged by the United States or any state on account of age.
Section 2. The Congress shall have the power to enforce this article by appropriate legislation.