You must be 18 years old to vote in a general election. In about half the states you can vote in a primary or caucus if you are 17 years old, but will be 18 by Election Day.
If you will be 18 years old on Election Day, you can register to vote. A growing number of states allow 16 and 17-year-olds to preregister so that they will start receiving information about voting as soon as they become eligible to vote.
You can find your state-specific regulations on the State Voting Requirements page. Choose your state, and on the following page select your election location, and "Eligibility Requirements."