If either Joe Biden or Donald Trump prevails in the November 2024 US elections, the next president will be the oldest in history to take the oath of office.
But while Biden's age and public slips and stumbles have been more widely scrutinised and examined, his blustery Republican rival's missteps are coming under sharper focus too.
In recent weeks, Trump has confused the Hungarian leader for the Turkish one, claimed that he defeated Barack Obama - and not Hillary Clinton, his actual rival - in the 2016 election, and warned that the world might be headed for a second, rather than a third, world war.
Rivals of the former president have been having a field day with such bloopers, quick to remind voters that Trump, the overwhelming favourite in Republican primary polls, is not so young himself.
Videos of the 77-year-old real estate tycoon stammering, verbally stumbling, or looking frail and sweaty have been gleefully posted on social media by the campaign team of Ron DeSantis. The Florida governor, a youthful 45, is currently leading Trump's Republican rivals in opinion polls in most states.
"WEAK AND FRAIL"
Biden's team, meantime, is not unhappy at being able, for once, to turn the spotlight on another ageing candidate - especially as the current president is turning 81 on Monday (Nov 20).
But the Trump camp has not taken such slights lightly.
"The Biden campaign must be confused like their own candidate because Biden is in a constant state of confusion," Trump spokesperson Steven Cheung told AFP.
In an unusually blunt and biting statement, Cheung went on to mock Biden as "weak and frail", prone to fall and even "just plain stupid".
Trump, during his campaign rallies, generally appears vigorous, interacting sometimes for hours with a sea of supporters.
But critics ask whether his repeated gaffes and the videos of him gripping a railing on a ramp to keep from falling, or trembling as he holds a glass of water, are reflections simply of fatigue - or signs of physical or mental decline.
From the outside, it is impossible to say.
While the two men are not far apart in age, Biden likely comes across as older, said political scientist Kyle Kondik.
The polls tend to agree: Two-thirds of Americans believe the current president is too old to serve a second term, while only half of those surveyed say the same about the slightly younger Trump.
It remains to be seen how the two will hold up once the campaign is fully underway, said Kondik. Rightly or wrongly, he added, Trump's verbal slips seem to be judged more leniently than Biden's, simply because Trump has made so many of them.
The Republican also does not face the same scrutiny over his physical condition as does his Democratic rival.
As US president, Biden - who is frequently seen biking with his Secret Service detail - faces an extensive series of annual medical tests, the results of which are reported in detail to the press.
While Biden's latest health summary described him as "healthy (and) vigorous", it also mentioned that a small lesion had been removed from his skin and that he had undergone a colonoscopy in 2021.
But since Trump left office three years ago, almost no details have been provided about the health of the former president, a known lover of fast food who appears to get little exercise except on the golf course.
The age of presidents was a matter of lively debate during the second term of Ronald Reagan, in the 1980s, when some observers speculated that he seemed to be showing signs of mental decline.
Years after he left office at age 77, it was announced that Reagan was suffering from Alzheimer's disease.
In 1994, former president Jimmy Carter himself sounded an alarm in an article in the Journal of the American Medical Association, warning of the "danger" to the United States if a president suffered a "neurological disease" that reduced his capacity for leadership.
The DeSantis campaign has increasingly been highlighting Trump's age and slip-ups.
"The president is not a job for an 80-year-old," DeSantis said Sunday on CNN.
Nikki Haley, a 51-year-old currently seeking the Republican presidential nomination, has repeatedly called for mandatory mental competency tests for politicians over 75.
But nothing of the kind appears likely anytime soon.