HOUSTON, TEXAS - SEPTEMBER 12: Democratic presidential candidate Sen. Cory Booker (D-NJ) speaks as South Bend, Indiana Mayor Pete Buttigieg looks on during the Democratic Presidential Debate at Texas Southern University's Health and PE Center on September 12, 2019 in Houston, Texas. Ten Democratic presidential hopefuls were chosen from the larger field of candidates to participate in the debate hosted by ABC News in partnership with Univision. (Photo by Win McNamee/Getty Images)
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HOUSTON, TEXAS - SEPTEMBER 12: Democratic presidential candidate Sen. Cory Booker (D-NJ) speaks as South Bend, Indiana Mayor Pete Buttigieg looks on during the Democratic Presidential Debate at Texas Southern University's Health and PE Center on September 12, 2019 in Houston, Texas. Ten Democratic presidential hopefuls were chosen from the larger field of candidates to participate in the debate hosted by ABC News in partnership with Univision. (Photo by Win McNamee/Getty Images)
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(CNN) —  

If you know one thing about last week’s presidential debate, it’s probably this: Julián Castro seemed to question Joe Biden’s age and mental acuity by repeatedly asking the former vice president if he was “forgetting” something he had said two minutes prior.

And that was important! But it’s what happened after the debate that actually might tell us more about where the race is headed – and how soon.

In an interview after the debate, New Jersey Sen. Cory Booker was asked by CNN’s Erin Burnett about Castro’s hit on Biden. And Booker responded this way:

“I think that we are at a tough point right now because there are a lot of people who are concerned about Joe Biden’s ability to carry the ball all the way across the end line without fumbling. And I think that Castro has some legitimate concerns about, can he be someone in a long, grueling campaign that can get the ball over the line. And he has every right to call that out.”

Which is, well, verrrrrrrry interesting. Here’s why.

Follow the trail of Democratic candidates raising questions of Biden’s competence. It started a few weeks back with Ohio Rep. Tim Ryan, a very long shot for the nomination (and that is being kind). “I just think Biden is declining,” Ryan told Bloomberg earlier this month. “I don’t think he has the energy. You see it almost daily. And I love the guy.”

Then, 11 days later, Castro, a more viable candidate but nonetheless still at the outer fringes of the race, picked up the baton and carried it with this attack on Biden’s health care plan during last Thursday’s debate:

“Are you forgetting what you said two minutes ago? Are you forgetting already what you said just two minutes ago? I mean, I can’t believe that you said two minutes ago that they had to buy in and now you’re saying they don’t have to buy in. You’re forgetting that.”

And then, immediately after the debate, Booker, an even more viable Democratic candidate, defended Castro and suggested that Biden may not be someone who can “carry the ball all the way across the end line without fumbling.”

It’s not hard to see what happening here. The attacks on Biden’s acuity are a) speeding up rapidly and b) being picked up by more and more viable candidates for the nomination.

Follow that path and you get this: Sometime soon, one of Biden’s main rivals for the nomination will raise – in an a-lot-of-people-are-asking-about-this soft sell – whether Biden is up to the rigors of this campaign and the prospect of taking on President Donald Trump next fall.

Biden has, to date, been able to dismiss the attacks on his abilities under the guise of not punching down. Responding to Castro’s questions about his competence would be to give the former San Antonio mayor exactly what he wants, and so Biden generally just ignores it. Ditto Ryan. And maybe even Booker.

So far, Biden has mostly talked in general terms about questions regarding his age and health.

“I think it’s totally appropriate for people to look at my age,” Biden said in New Hampshire earlier this month. “Just like when I was 29, was I old enough? And now, am I fit enough? I’ll completely disclose everything about my health. I’m in good shape.”

After the Houston debate, Biden answered a question about his health with a joke. “What health concerns, man?” he said to a reporter. “You want to wrestle?” (Biden’s campaign confirmed that he will release a full medical report before the Iowa caucuses, which are scheduled for February 3, 2020.)

It now appears to be a question of when, not if, Biden’s age and health become a central issue in the Democratic nomination fight. And my bet is sooner rather than later.