The 26th Amendment to the US Constitution was passed by Congress on March 23, 1971 and ratified on July 1, 1971, all during the Vietnam War. The amendment provided the right to vote to individuals who were eighteen years of age. Previous to this, the 14th Amendment had set the voting age at 21. Very strong feelings existed that if people were old enough to serve and die for their country, they should also be able to vote for those people sending them to war.
Text of the 26th Amendment
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.
The Congress shall have power to enforce this article by appropriate legislation.
Section 2 of Amendment 14 was modified by this amendment.