Twenty-sixth Amendment to the United States Constitution
During the 1960s, many Americans pushed both Congress and the state legislatures to lower the minimum voting age from 21 to 18.  This was mostly because of to the Vietnam War.  As the war went on, more and more people started to protest the war and become active.  During that war, many young men were drafted and sent to fight in the war before they were old enough to vote.  This meant they had no way to influence the people sending them off to risk their lives.  Protesters started to use the slogan "Old enough to fight, old enough to vote" again.