Over the past few years, many people have told me I should stop making Biden-is-old jokes because “it just helps Trump” — as if voters wouldn’t have noticed his age otherwise. I can’t ignore the obvious; none of us can. And I’m not going to mindlessly echo mendacious talking points like, “He has a cold.” If he had a cold, why was he out campaigning the next morning instead of putting his 81-year-old body to bed?

As Joe Biden himself would say, “Here’s the deal”: What happened at the debate last week wasn’t a tragedy, it was a blessing in disguise. I called on Mr. Biden to step aside almost a year ago, warning that he would be forever known as “Ruth Bader Biden” if he didn’t. Since then, each time I would bring up that idea, publicly or privately, people would dismiss it out of hand: Get on board, they’d say, the Democrats will never replace him, it’s off the table.

Well, now it’s on the table, where it always should have been. And far from being some kind of disaster for the Democratic Party, it plays right into what works best in 21-century American culture. Americans like new.

When Barack Obama announced he was running for president in 2007, many said he hadn’t been around long enough, not realizing that his youth and inexperience was one of the best things he had going for him. He was new, and we weren’t tired of him. And he didn’t have an endlessly long record to pick over.

If our presidential politics were a TV show, it would be a series past its prime in desperate need of new characters. The term “jumping the shark” derives from an infamous episode of “Happy Days,” when the Fonz literally jumped over a shark while water-skiing in his leather jacket. The show had been on so long, and the storylines had grown so tired, that the producers found themselves stretching the limits of reality to drum up interest.

Donald Trump talks about sharks a lot. I see this as an omen. Democrats can no longer afford to suspend disbelief.