For Joe Biden and Donald Trump - the two oldest candidates ever to seek the US presidency - age is an election issue neither can escape.

On Thursday, the current Democratic president, 81, and his Republican predecessor, 78, will face off in Atlanta, Georgia, for the first of two debates ahead of November’s vote, offering Americans a rare, split-screen comparison of the two men’s physical and mental strength.

For 90 minutes, under the glare of the high-definition cameras, President Biden and former President Trump — who remain nearly tied in national opinion polls — will spar on issues ranging from the economy and foreign wars to immigration and the future of democracy.

One slip-up, stumble or verbal miscue could cement concerns about their advanced age, with the potential for reshaping an already tight presidential race as voters begin to pay attention.

But delivering a vigorous performance may be more critical for Mr Biden, the nation’s oldest president who has been dogged by questions about his stamina and mental fitness since he took office.

“There’s no hiding the fact that Biden’s 81, there’s no hiding the fact that Trump’s basically the same age," said Jim Messina, a Democratic strategist who managed Barack Obama's 2012 presidential campaign. "It’s not a contest of age, it’s a contest of policy and character.”

“Part of what needs to happen on Thursday night is just to begin the conversation about the differences between them," Mr Messina said.

Polling shows that voters are far more concerned about Mr Biden’s age than his opponent’s. But if Trump wins, he would break Mr Biden’s record as the oldest president before the end of his term.

A March New York Times/Siena College poll suggested 73% of registered voters believed Mr Biden was “just too old to be an effective president”. Voters of all age groups expressed these concerns about the president’s fitness for office, including those 65 and older, according to the survey.

Just 42% of registered voters said the same about Trump, despite a mere three-and-a-half-year age gap.

“It ought to be about both of them, but Biden looks his age,” said Larry Sabato, the director of the University of Virginia Center for Politics.

Mr Biden was declared “fit for duty” earlier this year by a White House physician, but concerns about his age have persisted since he took office. Signs of his aging have become more notable in recent years, including a softer speaking voice, occasional memory lapses and a “stiffened gait” which his doctor partly attributes to arthritis.

As president, he is “covered almost every moment he is out in public” which means he endures more scrutiny than his challenger, Mr Sabato said.

Videos of routine actions - walking up and down Air Force One’s stairway, crossing a stage at public events — are watched closely online and in conservative media.

When Mr Biden tripped and fell at an Air Force Academy graduation in June 2023, the tumble made national news. After being helped to his feet, he continued to stand and walk as normal. His team said he had tripped on a sandbag on the stage, and the president later told reporters at the White House, “I got sandbagged!”

Some Democrats have publicly and privately expressed reservations about the president’s age, but they rallied around him in February, when justice department special counsel Robert Hur released his investigation into Mr Biden’s handling of classified documents after his term as vice president.

The report did not recommend prosecuting him, but Mr Hur’s description of the president as an an "elderly man with a poor memory” made headlines.

But when Mr Biden gave his annual State of the Union address a few weeks later, pundits gave him high marks for an energetically delivered speech.

“The president always delivers in big moments,” Congressman Ro Khanna, a Biden campaign surrogate, told NBC News last week. “He did in the State of the Union. And people are going to see the difference.”

The Biden campaign is hoping Thursday’s debate will be another moment in which the president demonstrates he can endure the rigours of governing, drawing a sharp contrast with Trump on policy and temperament.

Ahead of the debate, Donald Trump suggested his opponent could exceed expectations, telling the All-In podcast in a 20 June appearance that he assumed Mr Biden was "going to be somebody [who] will be a worthy debater”.

"I don’t want to underestimate him,” he added. Trump has separately spread unsubstantiated claims that the president will take performance-enhancing drugs to put in a good performance, which the Biden campaign has described as “desperate lies”.

While the scrutiny around Trump’s age is not as intense, the former president has faced questions about his own fitness for office.

At a January rally, Trump appeared to confuse his Republican primary rival, Nikki Haley, with former US House Speaker Nancy Pelosi, for several minutes during his speech.

He claimed, incorrectly, that Ms Haley had been “in charge of security” at the time of the 6 January attack on Congress. Ms Haley, Trump’s former UN ambassador, called for “mental competency tests” for politicians over 75 during her own unsuccessful presidential run.

Trump’s personal physician issued a statement in November attesting that his “cognitive exams were exceptional”.

At a Saturday rally in Pennsylvania, Trump complained of a double standard between the media’s treatment of himself and Mr Biden.

“If I say one word slightly out, they say, ‘He’s cognitively impaired,’” Trump told supporters. "Whereas Biden can run into walls. He can fall off the stage. He can fall up the stairs. He falls up.”

Both campaigns have sought to shape the narrative around their opposing candidate through social media, amplifying video snippets of verbal gaffes, memes and in some cases, deceptively edited footage.

More recently, Republicans and rightwing media have intensified attacks on Mr Biden’s mental competence, circulating a flurry of edited video clips including one of the president appearing to wander off during the G7 summit of world leaders in Italy. The unedited footage showed Mr Biden was walking to greet paratroopers during a skydiving demonstration.

Days later, conservative critics shared footage online of the president at a Los Angeles fundraiser, standing on stage before Barack Obama reaches for his arm and they walk off stage together. Donald Trump and other Republicans claimed it was evidence that Mr Biden had frozen up and had to be led off stage. But allies of the president pointed to longer clips that appeared to show Mr Biden smiling and taking in the crowd’s applause.

The Biden campaign has responded with a rapid response effort on social media, sharing content that appears to similarly raise questions about Trump’s mental acuity. They’ve posted clips of Trump appearing to walk off stage before he is supposed to and being redirected by others including former Vice-President Mike Pence and Israeli Prime Minister Benjamin Netanyahu.

Young voters won’t like what they see

Yet even as some of these videos are easily disproved, voters' reservations around Biden’s age continue to be one of his biggest vulnerabilities.

This is particularly true for younger voters, who are traditionally a more important demographic to Democrats than Republicans in national elections, organisers say.

Watching the debate could be an “eye-opening experience” for younger voters, said Amanda Litman, a Democratic strategist and founder of Run For Something, an organisation that trains and supports the next generation of Democratic candidates.

“I think it is indicative of a structure of politics that has privileged an older generation to stay in power for a long time,” Ms Litman said of the candidates' ages. “It is one of many reasons why, especially younger voters, feel disengaged. They don’t see themselves reflected in leadership.”

Brandt Williams, a 23-year-old accountant from Connecticut, said he will support Mr Biden again in 2024. He sees Mr Biden as a statesman who has a strong handle on foreign policy, and he supports the president's economic policies.

“The one issue is pretty common,” Mr Williams said. “His age.”

Though he planned to tune into Thursday’s debate, Mr Williams said he wished that he could vote for a “younger person, not only for vitality, but to represent all Americans, both young and old”.

In the critical battleground of Georgia, where the first debate will take place, Democrats told the BBC they sense apathy among their party’s young voters, with age a sticking point for many.

“Biden is stuck in a position where the younger generation wants action… they don’t want a speech about how we’re better as a nation,” said Titus Nichols, a 39-year-old attorney who is active in the Cobb County Democratic Party.

While the president “is the more mature, steady person, that’s not what people are looking for”, he added.

On Thursday evening, millions of people will be tuning in not only to hear the candidates’ ideas and arguments, but to judge the nominees’ stamina for themselves.

For Mr Biden, delivering a punch is critical if he wants to quell fears about his age, according to Ms Litman, the Democratic strategist.

“I think he needs to show competency and confidence,” she said. “And a little bit of fight.”