HOUSTON — Julian Castro went there.

The former housing secretary went after Joe Biden’s most sensitive liability in Thursday night’s Democratic presidential debate: Biden’s age and whether he has the mental acuity to beat President Donald Trump and lead the country.

While Castro’s attack was somewhat indirect — “Are you forgetting what you said two minutes ago?” he asked sharply and several times — it put a spotlight on an issue that Democratic voters have raised far more bluntly, even before Thursday’s debate, and one which is unlikely to go away, especially if Biden faces off against Trump.

Democratic presidential candidate former HUD secretary Julian Castro gives his closing statement Thursday, Sept. 12, 2019, during a Democratic presidential primary debate hosted by ABC at Texas Southern University in Houston. (AP Photo/David J. Phillip)
David J. Phillip / AP
Democratic presidential candidate former HUD secretary Julian Castro gives his closing statement Thursday, Sept. 12, 2019, during a Democratic presidential primary debate hosted by ABC at Texas Southern University in Houston. (AP Photo/David J. Phillip)

“Past his time,” Ned Reynolds, a Democrat and city councilman from Portsmouth, N.H., said of Biden in an interview days before the debate. "I think he’s too old; I think he’s too prone to gaffes and misstatements.”

Supporters dismiss the gaffes as a distraction and say Biden’s experience over more than 40 years in politics is an asset. Few of Biden’s rivals have so directly broached the topic, but have increasingly questioned his energy or sharpness, while denying they are implying anything about age.

“There’s a lot of people concerned about Joe Biden’s ability to carry the ball all the way across the end line without fumbling,” Sen. Cory Booker (D., N.J.) told CNN after the debate, arguing that Castro “has every right” to raise the issue. "I’ve listened to Joe Biden over the years and often felt like there are times where he’s going on or meandering in his speech. I want someone that can excite and energize.”

Both Booker and Castro later said they were not attacking Biden’s age, but U.S. Rep. Tim Ryan (D., Ohio), a presidential candidate who was not at the debate, said in the days leading up to it that Biden is “declining” and doesn’t have the “energy” to take on Trump.

Biden has fueled the argument with frequent gaffes on the campaign trail. He has mixed up anecdotes about soldiers, and his comments can veer into sometimes disconnected thoughts. He started Thursday night’s debate with vigor and a sharp (and likely rehearsed) attack on his rivals’ health-care plans.

But at one point later, he urged parents to “make sure you have a record player on at night” so children hear more words and build their vocabularies, and, in an answer about racial inequality, careened into comments about relations with Venezuela.

He ended the previous debate by bungling an appeal for supporters to text message his campaign (a way of gathering information).

If elected, Biden would be 78 when sworn in, making him the oldest first-term president in American history, surpassing Trump, who is now 73 and often the subject of attacks questioning his mental stability.

Sanders is two years older than Biden and would be 80 on Inauguration Day. Warren would be 72, but she and Sanders rarely stumble like the former vice president does. Both have won praise from liberals for newer ideas.

Rebecca Kirszner Katz, a progressive strategist unaffiliated with any of the campaigns, said she thinks it’s Biden’s more traditional, moderate policies that might repel some voters.

“It’s not his age that bothers many Democrats, it’s how outdated some of his ideas are,” she said. Still, she thinks those occasional gaffes are worth paying attention to. “That’s what this whole primary process is about — ability. Who has the ability to compete in this atmosphere?”

Yet it’s unclear how much his errors are related to age or just Biden being Biden. Even decades ago he was known for his penchant to put his foot in his mouth. A recent CBS/YouGov poll of Democrats in 18 early voting states found that 31% thought Biden was too old to be president, against 37% for Sanders and 5% for Warren.

It’s also unclear how much voters care about verbal missteps. In interviews with active New Hampshire Democrats last week, the age factor cut both ways. Several said his longevity is an asset.

When it comes to the key issues Democrats are talking about, “Biden has already done it in his career, in his career as a senator, as a vice president," said Sumathi C. Madhure, 58, of Nashua.

For those who say Biden, 76, is past his prime, and point to his sometimes fumbling speech, others praise his experience and see his misstatements as a sign that he’s a regular guy. For those who worry he won’t have the energy to excite voters or withstand Trump’s withering personal attacks, there are those who see a familiar face who can offer stability after the turmoil of the current president.

“I trust him. I don’t know if he’s going to lead a social movement, but I trust him to get us through the next four or eight years,” said Josh Altman, a 29-year-old Harvard graduate student who votes in Georgia and was considering Biden as one of his top options.

Yet some older voters, reflecting on their own experiences with aging, questioned whether Biden could do the job. “We’re in his age group. It’s a job for a younger person,” said Pam Kirby, a 67-year-old from Portsmouth.

Some say Biden’s gaffes don’t compare to some of the offensive things Trump said ahead of being elected president, including attacking the parents of a slain soldier and being caught on tape bragging about groping women. Biden has remained atop Democratic polls despite a number of flubs.

Raising the age issue at all is also fraught for Biden’s rivals, even if they know Trump won’t be so delicate. (At his own event Thursday night Trump again called Biden “sleepy Joe” and said, “He has no idea what the hell he’s doing or saying.”) The former vice president retains significant goodwill in the party, and, as his campaign aides pointed out after the debate, people who have attacked him have seen little to no gain in polling.

U.S. Rep. Brendan Boyle, a 42-year-old from Philadelphia who supports Biden, noted that the three top Democrats in the House, including Speaker Nancy Pelosi, are all older than 70. “I don’t see any difference in Joe today vs. 10 years ago,” he said.

Another Philadelphia supporter, U.S. Rep. Dwight Evans, said voters in his heavily African American district care only about beating Trump.

"I don’t think what anyone says in a one minute debate clip will really shake support for what people know he is capable of doing,” Evans said. His constituents, he said, "don’t even know who Julian Castro is. I’m just being frank with you.”