You might find this interesting, from www.usconstitution.net;
The Right To Vote
The Constitution contains many phrases, clauses, and amendments detailing ways people cannot be denied the right to vote. You cannot deny the right to vote because of race or gender. Citizens of Washington DC can vote for President; 18-year-olds can vote; you can vote even if you fail to pay a poll tax. The Constitution also requires that anyone who can vote for the "most numerous branch" of their state legislature can vote for House members and Senate members.
Note that in all of this, though, the Constitution never explicitly ensures the right to vote, as it does the right to speech, for example.