54. the answer is...

eighteen (18) and older

For most of U.S. history, Americans had to be at least 21 years old to vote. At the time of the Vietnam War, during the 1960s and 1970s, many people thought that people who were old enough to fight in a war should also be old enough to vote. In 1971, the 26th Amendment changed the minimum voting age from 21 to 18 for all federal, state, and local elections. The National Voter Registration Act of 1993 made it easier for people to register to vote. Now they can register to vote by mail, at public assistance offices, or when they apply for or renew their driver’s license.