Amendment 26

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.

This information is from  http://www.law.cornell.edu/constitution/amendmentxxvi

This amendment gives the right to vote at the age of 18 because I guess the government thinks the age of 18 is when you can make your own rational decisions.