The right to vote is an interesting facet of constitutional law, given that the constitution does not explicitly grant it. What the Constitution does do, through various amendments, is prevent the government from restricting the right to vote because of certain factors. For example, the Fifteenth and Nineteenth Amendments state that the government cannot deny someone the right to vote because of their race or gender, respectively.
The Twenty-Sixth Amendment changed the standard voting age in all elections to 18.
"The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age. The Congress shall have power to enforce this article by appropriate legislation."
United States Library of Congress, The Constitution of the United States of America: Analysis and Interpretation
In extending the Voting Rights Act of 1965 in 1970,1 Congress included a provision lowering the age qualification to vote in all elections, federal, state, and local, to 18.2 In a divided decision, the Supreme Court held that Congress was empowered to lower the age qualification in federal elections, but voided the application of the provision in all other elections as beyond congressional power.3 Confronted thus with the possibility that they might have to maintain two sets of registration books and go to the expense of running separate election systems for federal elections and all other elections, the states were receptive to the proposing of an Amendment by Congress to establish a minimum age qualification at 18 for all elections and ratified it promptly.4