It would require voting for the other party. The big issue is the parties being willing to back aging candidates. But of course, party leaders are just as old -- so they see no problem.
California runs an open primary and a top two general.
Due to practicality and congressional gridlock, this has to happen state by state, and in many states the 'easiest' method is some sort of ballot initiative. I encourage anyone who reads this to look up the process in their state (https://ballotpedia.org/List_of_ballot_measures_by_state) and at least try to get a signature out to one so they show up on the ballot.