Former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.) defended themselves this week against suggestions that they may be too old to run for president, with both candidates seeking to bolster their case for taking on President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE next year.

Biden, 76, and Sanders, 78, are among the oldest candidates in the crowded 2020 presidential field — they both surpass Trump, who is 73 — and they are top Democratic contenders for the White House, along with Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.).

But they have both also faced questions about their age — Biden virtually since he launched his campaign, with questions exacerbated after a series of gaffes and halting debate performances; and Sanders due to his position as the oldest candidate in the field, with concerns thrust into the spotlight after a heart attack this month that briefly took him off the campaign trail.

Biden and Sanders have sought to flip the narrative surrounding their age, arguing their tenures in public service show that they have experience to handle the job of president. Biden often touts his time in the Obama administration, while Sanders points to his past as a civil rights advocate.

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“What they're really trying to make the case is about age. And with age comes experience, with experience comes wisdom, and with wisdom comes judgment,” Biden told CBS in an interview set to air Sunday when asked about concerns that he would be able to go toe-to-toe with Trump at a debate.

During a press conference in Iowa on Friday, Sanders said that "the ideas that I am fighting for now didn’t come to me yesterday."

“I’ve been on more picket lines, I expect than all my opponents combined over the last 30 years,” he said.

“Having a long record gives people the understanding that these ideas that I am talking about — they are in my guts. They are in my heart,” he added. “This is who I am as a human being, and it ain’t gonna change.”

Both candidates, who would each be the oldest president ever at the start of their terms, have sought to reassure voters they have the capacity to see their campaigns through to the general election and beyond.

Yet despite their defenses, both candidates appear to have room to improve with voters who remain concerned about their ages.

Twenty-eight percent of likely early-state Democratic primary and caucus voters said Biden is too old to effectively serve as president, and 43 percent said the same of Sanders in an October CBS News/YouGov poll.

Warren, who is 70 years old and is the other top-three primary contender, was able to keep that figure down at 4 percent.

While questions about Biden's and Sanders’s ages have percolated among voters, a debate about the two septuagenarians’ fitness for office has barely registered as a concern in remarks among the 2020 primary candidates.

The issue of age was pushed into the spotlight this month after Sanders was forced off the campaign trail before the latest Democratic debate due to a heart attack, while Biden also swatted back at rivals who went after him in a debate in September.

During that debate, Housing and Urban Development Secretary Julián Castro was seen as taking a below-the-belt shot at the former vice president by insisting that Biden contradicted himself when he said some people would have to buy into a health care plan under his proposed system.

“Are you forgetting what you said just two minutes ago?” Castro asked Biden, sparking gasps in the crowd.

Experts have warned that going negative against either candidate could backfire, pointing that while concerns exist, Biden and Sanders continue to have enough support to poll at or near the top of almost every survey.

“If you go really negative, people are going to remember you and they’re not going to like you,” Howard Dean, the former Vermont governor who unsuccessfully sought the Democratic presidential nomination in 2004, told The Hill last month. “These people are at the top of the polls because people like them.”