Joe Biden is an old man. On the campaign trail, the 76-year-old Democratic presidential frontrunner comes across as strong and healthy in a way most men his age might envy. When he sits for interviews with late-night talk show hosts, he can seem polished, even presidential. But he can also stumble over his words or seem to forget things, as he did when he botched an attempt to tell people to text a number if they supported him during a Democratic debate, or when he appeared to forget Barack Obama's name during a speech. Those haven't been his campaign's only awkward moments. "It is impossible not to notice that Joe is getting old," Hamilton Nolan wrote in Splinter last month after attending an Iowa campaign event. "Biden is not senile, or unable to function, but the signs of the slow creep of cognitive decline are too visible to ignore."

Even a young Biden was prone to "gaffes," a political-journalism euphemism that encompasses everything from verbal flubs to exaggerating one's personal history to expressions of prejudice. So it's unfair to chalk up all his campaign-trail flubs to his age. Yet speculation about Biden being less sharp than he used to be—and worries that Donald Trump will weaponize those accusations should Biden become his party's nominee—is running rampant among those watching 2020 closely.

This focus on Biden's age seems unfair for another reason, however. Trump himself is 73. Bernie Sanders, another prominent 2020 contender, is about to turn 78. Elizabeth Warren is 70. If voters are worried that Biden is too old to take on one of the most important and mentally challenging jobs in the world, should they not be equally concerned that nearly all of the other applicants are in their eighth decade of life?

More bluntly: Is there such a thing as being too old to be an effective president?

AARP, the country's leading advocacy group for senior citizens, thinks that sort of question reeks of age-based discrimination. "In the case of just about every other prejudice—e.g., based on race, ethnicity, sexuality, gender, disability, etc.—reasonable people are at least aware of the idea that making sweeping judgments based on such identifiers is not okay. But ageism still gets a pass," AARP CEO Jo Ann Jenkins said in a statement. She added that while some older people "slow down," experience and accumulated wisdom are vital for a job like the presidency. "A political leader, or CEO, or writer, or teacher, or thinker (the list goes on) of the same age might very well be the best they’ve ever been, simply because they’ve got more to draw on."

Elizabeth Zelinski, a professor of gerontology and psychology at the University of Southern California, said it's true older people are generally better at some things than younger people, in particular "divergent thinking," the ability to approach problems in creative or unexpected ways. But when it comes to some of the mental skills most important to the presidency—memory, attention span, and "executive function," which means collating information to make decisions—older people tend to be worse than they used to be. "These abilities, when they are measured in isolation, decline with age," Zelinski said, a process which generally begins after age 60. (Educated, healthy, and active older people like all of the presidential candidates, she added, are less likely to decline.)

A 2014 paper in Politics and the Life Sciences titled "Executive Dysfunction, Brain Aging, and Political Leadership," laid out the case to be worried about the age of our leaders. The prefrontal cortex, the region of the brain most associated with executive function, is also "the specific brain region showing the greatest age-related volume-loss in healthy individuals," the authors report. And this decline may not be obvious: "While an individual may appear to be normal from the perspective of overall cognitive function, that same individual may have an impaired ability to integrate basic cognitive skills such as language and memory, in order to achieve normal decision-making capacity."

Political leaders do not routinely subject themselves to brain scans or other types of advanced tests, but the paper's authors were clearly concerned about older leaders in general. "Based on the known neuroanatomical and neuropathological changes that occur with aging," they write, "we should probably assume that a significant proportion of political leaders over the age of 65 have impairment of executive function."

That's a particular problem for the U.S. Not only are the leading contenders for the White House (with the exception of 54-year-old Kamala Harris) over 65, but many congressional leaders from both parties are over 75. When asked if people should be concerned about the advanced age of the top tier of the political class, Mark Fisher, the University of California, Irvine neurology professor who was the lead author on that paper, said, "This is a very legitimate question, and should not be dismissed as 'ageism.'"

AARP spokesperson John Hishta said the age of a presidential candidate is not, and should not be, the focus of voters as the 2020 campaign ramps up. "I have seen no data anywhere among voters—whether it's in the primary or general election—where this is an issue that is of concern to them," he said. "They're more interested in what affects them in terms of economic security and healthcare and issues like that." (One recent poll found that Democratic voters overwhelmingly didn't want a president in their 70s, though it's hard to know what to make of that result given that most Democrats seem to support over-70 candidates.)

Donald Trump is the only president who was in his 70s on the day of his first inauguration, but the current crop of candidates suggests he won't be the last. If the U.S. is going to be run by someone beyond the age at which most of us begin to fade, however slightly, it seems likely questions about how to measure decline will become more important.

Presidential candidates and presidents have long worked to reassure voters they're hale and hearty. Not wanting to seem sickly may have driven Hillary Clinton, who was 68 at the time, to attend an outdoor 9/11 remembrance ceremony in 2016 despite having pneumonia, where she stumbled and was held up by aides. Some presidents have kept much more serious health problems a secret from the public, most notably Woodrow Wilson, whose wife essentially took over for him after he had a stroke near the end of his term. Public concerns about the health of the president is partly why they traditionally release the results of their physicals. When Trump aced a test that measured neurological decline last year, the White House trumpeted his success, surely in no small part because the administration knows Trump's mental agility is a matter of debate.

"It would be nice if we had measures of his brain function, like neural imaging measures, but we don't have methods that go back far enough to be able to trace change," said Zelinski, the USC professor, who noted that you ideally want to have access to past tests to measure how and whether an individual is declining. "The thing about cognitive change in people is that it's a pretty slow process unless they are developing dementia."

According to Dean Keith Simonton, a professor emeritus of psychology at UC Davis, standard tests like the one Trump took are meant to pick up major cognitive disabilities due to strokes, dementia, and brain tumors. "We don't want to set the threshold at that level, surely," he said. Fisher said the test "is relatively easy to 'game'" if you prepare for it. "Ideally, a combination of more comprehensive neuropsychological testing and brain magnetic resonance imaging would give some clear-cut answers," Fisher added.

Zelinski thinks we should weigh more seriously the cognitive abilities of people whose decisions could affect the lives of millions—or billions. Specifically, she argued, cognition should be assessed along with other routine health matters during presidential physicals. And more broadly, society should think about the soundness of other figures who have authority. "There is no fitness-to-serve test for CEOs, for people who are in positions of great power, and maybe there should be," she said. "I don't think it's a terrible thing to assess that."

Some presidential candidates might voluntarily submit to a battery of tests to reassure voters they are up to the job. (Zelinksi said they'd have to be careful to make sure the tests were conducted by a neutral party to avoid charges of bias.) But judging cognitive ability based on campaign-trail gaffes, experts consulted by VICE agreed, is a fruitless, unscientific exercise. Instead of constructing a narrative out of a handful of incidents, a la Trump's odd statements or Biden's flubs, observers should look for times when candidates have to speak unrehearsed at length, like in a debate.

Simonton pointed to a 1988 paper that took a method devised by psychiatrists to analyze speech transcripts for signs of cognitive decline and applied it to the 1984 presidential debates. Among the candidates, Reagan had the highest cognitive impairment score—which, the paper notes, wouldn't have surprised any of the viewers who watched. Simonton called such a method a possible way to judge cognition "without interjecting political bias," always a threat when discussing polarizing figures.

But 35 years after those debates, there still isn't an agreed-upon way to judge whether a presidential candidate might be losing their edge, even as the presidency has become a more important and difficult job and U.S. political leaders have increased in age. There may be no way to tell whether a president is in decline until he or she has already fallen off a cliff.

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