Actually he would have only been 18yo at the time, not even old enough to vote in 1965 (you had to be 21).
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.