From Action Vote.com
The history of voting in America is a story of ever-increasing voting rights. The rules for eligibility have changed substantially since America's founding, and continue to change today. When America was young, only white males over the age of 21 were allowed to vote. Some of the landmark changes since then:
The 13th, 14th, and 15th Amendments to the U.S. Constitution were passed following the Civil War, in the later 1860s. They outlawed slavery and extended civil rights and suffrage (voting rights) to former slaves. The LEGAL right to vote for African-Americans was established, but numerous restrictions kept many blacks from ACTUALLY voting until the 1960s Voting Rights Act.
Direct election of Senators
The 17th Amendment made it so U.S. Senators were directly elected by popular vote. Prior to 1913, Senators were appointed. The President, of course, is still not elected by popular vote, but by the Electoral College. For example, in the presidential election of 2000, Al Gore won the popular vote but George Bush won the electoral college vote.
The 19th Amendment gave women the right to vote in 1920. This amendment resulted from an international movement of "Suffragettes". Women still lacked the right to vote in Switzerland until the 1970s, and as of 1990 women could not vote in Kuwait.
The 26th Amendment lowered the voting age from 21 to 18. This occurred in 1971, amid the Vietnam War, when 18-year-olds were routinely drafted and sent to war without the right to vote.