The U.S. Supreme Court has never had a case involving the 26th amendment, which says, “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 any State on account of age.”
On December 7, some Texas voters asked the U.S. Supreme Court to hear their case against the Texas law that treats voters age 65 and above differently than it treats younger voters. Texas lets voters age 65 and above cast a no-excuse absentee ballot, but does not permit any other voters to do so. Cascino v Nelson, 23-612. Here is the cert petition.
A few years ago a similar Indiana law was brought before the U.S. Supreme Court. The Court asked for a response from the state of Indiana, but then afterwards the Court refused to hear the Indiana case, which was Tully v Okeson, 20-1244. Thanks to Democcracy Docket for the news about the Texas case.