The most notable exchange in Thursday night’s Democratic debate, in which former housing secretary Julián Castro seemed to allude to former vice president Joe Biden’s mental sharpness, may or may not have opened the floodgates on a topic that has heretofore been kept in the background of the Democratic race: whether Biden, who is 76, is up to being president.

Now critics and even the media are debating the fairness of pointing out Biden’s age — and the challenges that may present. (Castro, after the debate, said he was “fact checking” Biden. He also said Friday that his questioning Biden’s memory had nothing to do with the former vice president’s age.)

The Fix’s Aaron Blake wrote last month about Biden’s recent, frequent flubs on the campaign trail and why they matter to his candidacy. That context was the backdrop for when, in a back-and-forth over health care, Castro accused Biden of not remembering what he had said moments before.

“You just said that. You just said that two minutes ago. You just said two minutes ago that they would have to buy in,” Castro charged.

There were audible groans in the audience. That Castro was incorrect about what Biden had said didn’t help his argument, and it probably further muddles the debate over whether a candidate’s age is fair game.

The blowback continued after the debate in the spin room and on social media.

Former White House ethics lawyer Richard Painter found Castro’s comments so unacceptable that he suggested the former San Antonio mayor end his bid for the presidency.

Sen. Amy Klobuchar (D-Minn.) defended Biden on CNN.

“I just thought that ‘this is not cool,’” she said before arguing that President Trump has made personally attacking opponents the new normal in presidential campaigning. “I thought that was so personal and so unnecessary.”

Sen. Cory Booker (D-N.J.), who has been willing to attack Biden on his record plenty, though not on this issue, was the only other candidate who backed up Castro. “I think we are at a tough point because there are a lot of people who are concerned about Joe Biden’s ability to carry the ball across the finished line without fumbling,” he said on CNN after the debate. “And I think Castro has some legitimate concerns.

“He has every right to call him out,” Booker added.

When Castro was asked in follow-up interviews about bringing up a question that implicated Biden’s age, he tried to say he did not.

“Number one, he couldn’t hear me in the auditorium,” Castro said of Biden. “Secondly, look, that’s a question that I would ask any opponent in a debate.”

“The vice president, is, you know, I mean, he’s been around for a long time. Okay, when we’re up there, we’re up there to debate,” he added. “It didn’t matter that it was Joe Biden. If it had been another candidate who tried to deny what they said just two minutes ago, I would have asked them the same thing.”

While the candidates deliberated on the aftermath of the elephant in the room having been addressed, on social media, the conversation turned to the appropriate way to do so: What’s legitimate criticism or areas for concern when it comes to a 76-year-old man who seems to frequently stumble over words and facts — where does that veer into ageism?

A lot of people consider Biden’s mental sharpness — and his argument that he is the best candidate for the job — fair game. Thursday was not the first time that questions about the former vice president’s acuity have arisen.

At the New Hampshire Democratic Party convention in August, Rep. Tim Ryan (Ohio), who is also running for president but was not in Thursday night’s debate, appeared to allude to Biden’s sharpness.

“It is a concern you’re hearing from a lot of people in the country,” he said. “I’m just saying that it’s unclear sometimes ― when [Biden] is articulating positions, there’s a lack of clarity. And I’ll leave it at that.”

But plenty of people on Twitter also questioned whether attacking Biden the way Castro clearly did — alluding to his age — was fair game.

April Reign, an activist who focuses on diversity and inclusion issues, argued that Democrats critical of Castro never would have had a problem with the line had it not been used on the former vice president.

“Similarly, if Biden (or any other candidate) had used Castro’s ‘two minutes ago’ line on Trump, every Democrat in America would be calling it a strong line,” she tweeted. “If you want someone tough to go after Trump, they have to prove their mettle against their fellow nominees. Grow up.”

Other liberals said the response to Castro — especially from establishment Democrats and leaders in the mainstream media — was revealing of something else. Preston Mitchum, who teaches health law and policy at Georgetown University Law Center, told The Fix that Castro’s response to Biden in a conversation about health-care policy appeared to be about silencing valid criticism.

“I think it’s a little bit layered when you think about it,” he said. “Castro is Latinx. There is a tendency among some white people to believe that there is a particular way that people of color should respond to older people, to white people and in particular to older white people, so I was not surprised when I saw the reaction of folks who said that he was ‘nasty.’”

That could certainly be part of the dynamic. But Castro’s question may have opened the floodgates on a topic that the media and other candidates have been reluctant to tackle head on. Now that it has been broached, it will be interesting to see if either are more willing to keep going there.