For months, they’ve been waiting for Joe Biden to collapse. Old Joe. Sputtering Joe. The Septuagenarian Slug. He’s That Seventies Show. The woke Democratic caucus waits for him to forget a name, or mistake his cellphone for the remote. Ditto the woke press. He mentioned “record player,” for God’s sake, answering a question about racism!

Keep waiting. In a debate where Democrats finally went there on guns, saying what most Americans already believe — “Hell, yes, we’re going to take your AR-15, your AK-47,” said Beto O’Rourke, in the line of the night — they finally went there on the big Biden question: his age.

The takedown couldn’t come from fellow 70-somethings, Sens. Bernie Sanders or Elizabeth Warren. Sanders’ voice was in the key of gravel, making his Brooklyn accent more distracting than normal when he shouted, “We can do mo-wah!” And Warren was on her best behavior.

So the dirty work fell to Julián Castro, the former Obama Cabinet member. He got right to it with an opening jab against “old ideas,” and then a direct slap, asking Biden: “Are you forgetting what you said two minutes ago?” Um, no.

It was a big backfire, a cheap shot that fell to the floor. But it did finally bring out into the open the “ism” that dare not speak its name. You’ll hear plenty more of it, much of it coming from the infantile 73-year-old president, who, just this week, seemed to forget that he had a son with his third wife.

Age itself is not the problem. Winston Churchill was in his mid-70s when he became, for a second time, Britain’s prime minister. Konrad Adenauer served as chancellor of postwar Germany well into his 80s. And how old was Nelson Mandela when he took office as South Africa’s first black president? He was 75.

But age will be the weapon that Biden’s opponents will use, because it’s basically all they have.

Biden is running as a normal guy. Normal is dull. It stirs no blood. Normal is not having to worry about who’s piloting the plane; it’ll land. Uncle Joe is comfort food, the “Home is the place where, when you have to go there, They have to take you in,” in Robert Frost’s words.

But can normal be transformative? Is it enough to say that our long national nightmare is over, as people work through the post-traumatic stress syndromes that’ll be left behind by Donald Trump?

Biden’s persistent big lead in the polls over Trump is your answer. So what if he repeats “battle for the soul of America” in framing the election over and over. It is. In the Trump era, a majority will choose avuncular over authoritarian any day.

Biden took his energy drink, or something. He came out on fire. He was sober, direct, returning volleys from the two lefties on either side of him. “For a socialist, you have a lot more faith in corporate America than I do,” he said to Sanders.

But a half-hour into this third Democratic debate, many Democrats had to be wondering: Can he go three hours? It’s our own collective ageism.

He did, closing with his most appealing side: his humanity and vulnerability. After the El Paso shootings, Trump showed he couldn’t even pass the base test of a human being, demonstrating a smidgen of empathy. He couldn’t even fake it. Not for a minute. Biden bleeds empathy. What’s appealing about him is what my colleague Jennifer Senior called “the bartender aspect” of his character, in her review of his book on the loss of his son Beau.

Progressives looking for someone who will bring big structural changes should remember that we are electing not just a president, but an executive branch. I have no doubt that Biden would sign any kind of advanced legislation that can get by the Neanderthals holding the Senate back in the last century.

Waiting for Uncle Joe to collapse is not a winning strategy. Most of the other candidates in Thursday’s debate seemed to get that. O’Rourke had his best night. This is the rare politician who’s not afraid to let it rip, the b.s. valve turned off. Kamala Harris is brilliant and would crush Trump in a debate. “You can go back to watching Fox News,” was her line that should be on bumper stickers soon. And on the shootings, she said of Trump: “He didn’t pull the trigger, but he’s certainly been tweeting out the ammunition.”

For the two of them, though, it may too late: both Hillary Clinton and Donald Trump led at this stage in the polls four years ago.

Pete Buttigieg, born in 1982, nearly a decade after Biden entered the Senate, makes a pitch for generational change, citing John F. Kennedy and Barack Obama. He’s got the right song in the wrong presidential year. Remember the guy who specifically called for passing the generational torch in an earlier debate? Me neither. (It was Rep. Eric Swalwell.)

Cory Booker, another man of reason and intellect stuck in a politician’s body, inadvertently gave Biden some cover, the raison d’être for his candidacy. “We’ve got one shot to make Donald Trump a one-term president.” That’s the calculation most Democrats are making as well, and why Biden is the likely nominee.