The voting age for federal elections was lowered to 18 by a constitutional amendment (the 26th), but I don’t see how a state could be prevented from making the voting age 17 to vote for electors for President or for members of Congress, as long as the same age was required to vote for the most numerous branch of the state legislature.
“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”
The age is 18