Amendment XXVI:

Section 1.

The right of citizens of the United States, who are 18 years of age or older, to vote, shall not be denied or abridged by the United States or any state on account of age.

Section 2.

The Congress shall have the power to enforce this article by appropriate legislation.

If you are 18 you are able to register and vote and cannot be stopped. 

These pictures both show signs of being able to vote at the age of 18!