The 26th Amendment to the U.S. Constitution lowered the voting age to 18 in all states. The Amendment passed Congress on March 23, 1971 and was ratified by the states as of July 1, 1971.
On June 22, 1970, President Richard Nixon signed a law lowering the voting age to 18 for all elections -- federal, state, and local -- but he questioned whether or not that law was constitutional. When Oregon and Texas challenged the law in court, the Supreme Court supported the states, ruling in Oregon v. Mitchell, 400 U.S. 112 (1970), that Congress had indeed overstepped its powers by attempting to lower the voting age in state and local elections.
Congress went to work on the constitutional amendment soon after -- in large part because of the Vietnam War, in which young men who were not allowed to vote were nevertheless drafted to fight. The slogan, Old enough to fight, old enough to vote, coined during World War II to encourage individual states to lower their voting ages, neatly summed up the growing controversy. After the Amendment was drafted and proposed by Congress, most states fell in line with it; no constitutional amendment has ever been ratified by the states so quickly.
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.
The Congress shall have the power to enforce this article by appropriate legislation.