Which amendment extended voting rights to citizens who are 18 years old?

The 26th Amendment to the U.S. Constitution specifically extends voting rights to citizens who are 18 years old. Ratified in 1971, this amendment was largely influenced by the argument that if individuals at 18 years could be drafted into military service, then they should also have the right to vote. This amendment aimed to ensure that younger citizens had a voice in the political process, reflecting a significant shift in societal views regarding the age of adulthood and participation in democracy.

The other amendments mentioned do not pertain to voting age. The 24th Amendment addresses the prohibition of poll taxes in federal elections, the 20th Amendment defines the terms of the President and Congress, and the 17th Amendment provides for the direct election of U.S. Senators. Thus, the 26th Amendment is the one that directly identifies and establishes voting rights for citizens who reach the age of 18.