Since either of them winning would mean a term limited incumbent in 2028, my guess is that both parties will nominate a Millennial candidate for the next election, both messaging that they're the party of fresh ideas for the future.
The parties barely have control over who gets nominated, no one in the Republican Party thought Trump was a good idea in 2016 at the start of the primaries. It only matters who gets votes, and the Republican side skews old enough that a millennial would have problems.
