No one said "federal" elections. Here's proof for state and local elections:
http://www.abanet.org/publiced/lawday/convo/00/citizenship.html
Old Enough to Fight, Old Enough to Vote: Some History
“Old enough to fight, old enough to vote,” was a slogan that was first heard after the United States entered World War II in 1941. Georgia led the way in 1943, by lowering the voting age from 21 to 18 in state and local elections. Millions of men and women too young to vote were asked to serve in that war. Many people, regardless of age, thought it was unfair. However, it wasn’t until the U.S. was involved in the Vietnam War, thirty years later, that the movement to give eighteen-year-olds the right to vote gained strength.
Oops, I should've said:
No one said "national" elections.