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.
Then twenty-sixth amendment set the minimum voting limit to be no more than eighteen years of age.
Here is a clip from the twenty-sixth amendments certification ceremony.
Here is a man stating the effects of the twenty-sixth amendment.