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.
There's more than two parties. Nobody is forcing you to vote for one of them. Parties are also an anti-pattern. Again we need a better system than first past the post that naturally biases towards a two party system.
Party primaries devolve into 'the devil you know vs the devil you dont' and the Devil You Know usually has a boatload of cash to spend on the campaign that the Devil You Dont doesn't have. You are right of course, the solution lies in eliminating the party system and implementing a ranked voting system of some kind.
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.
California runs an open primary and a top two general.