Joe Biden entered national politics as one of the youngest people to ever serve in the Senate. Today, at 76, he’s considering a run for president which, if he clinches the nomination, would make him one of the oldest major-party nominees to vie for the office. But he wouldn’t even be the oldest candidate this election cycle. With 77-year-old Bernie Sanders also seeking the Democratic nomination and 72-year-old President Trump in the White House, Americans are looking at a veritable Boca Raton of septuagenarian candidates. Some media outlets are starting to ask — how old is too old to be president?

To which this media outlet responds: You can ask, but good luck finding a straight answer.

We’ve written before about old candidates; during the 2016 election, we looked at what age could portend for the physical health and life expectancy of the people vying to occupy the White House. We spent less time on how age affects the brain. Everybody experiences some amount of cognitive decline as they enter their final decades. We think a little more slowly. We have a little more trouble remembering facts, or filtering the things we want to say but shouldn’t. Doctors can measure these losses, but what those measurements should mean for a presidential candidate is harder to pin down.

Getting older definitely changes the way we think. And while the risks of diagnosable dementia go up with age, it’s the changes that fall short of real illness that are more likely to create ethical dilemmas in a work environment, said Paul Moberg, a professor of neuropsychology at the University of Pennsylvania. The grey area between “senior moments” and the early stages of Alzheimer’s is wide enough to encompass people whose impairments might be tough to spot unless you’re a close friend (or a doctor performing a cognitive function test). Often, Moberg said, the person themselves is the last to know that anything has changed in their brain.

Meanwhile, there’s no universal standard for how much change a single person can experience before it starts to get in the way of doing their job.

As a result, it’s normal for people working in high-stakes professions to undergo routine cognitive testing once they’re over a certain age. Moberg’s team administers tests like these, which evaluate things like verbal skills, memory, concentration and visual-spatial understanding. They’re designed to spot low levels of cognitive decline — a level of change that could affect the work of an aging doctor, pilot or lawyer. Moberg himself will have to take one someday. “Once you pass 70, [the University of Pennsylvania] requires a cognitive assessment to continue,” he said.

Because of tests like these, we know that it’s very normal to lose some brain power as you age. Average losses differ by demographic factors — in general, the more education you have, the better you’ll do compared to people of the same age range. But across the population as a whole, everybody’s scores decline as they get older. For example, in a 2011 study of more than 2,500 people taking the Montreal Cognitive Assessment — a 10-minute test that measures brain function by having subjects draw clocks, remember lists of words and perform other simple tasks — 30-somethings with at least a few college credits under their belt had an average score of 25.81 out of 30. In contrast, the average score of a septuagenarian with the same education level was 23.6.

But knowing that age comes with a slow-but-significant decline in your score on an abstract test isn’t the same thing as knowing how old is too old to be president, said Jacob Appel, a professor of psychiatry and medical education at the Icahn School of Medicine at Mount Sinai who studies both bioethics and the mental health of past presidents.

There are a couple of problems here, Appel told me. A test score can give a neurologist some idea of how to help a patient live their best life, but it can’t tell anyone whether they’re qualified to hold public office. And if the test scores were made public, it would be very easy for laypeople to misinterpret numbers as hard, universal cutoffs. “You’d have to send the public to medical school to get them to understand,” he said. “And I’m not even sure whether a 26 or 25 would make you unfit for the presidency.”

Likewise, Appel said, by the time someone is highly educated and skilled enough to run a plausible campaign for the White House, they also could conceivably experience a little decline in function without it affecting their overall assessment score. Someone who scored a 30 on the cognitive assessment test at age 40 could still be hitting a 28 at age 70. They might be fuzzier than they used to be, but their version of fuzzy might still be sharper than most people. Whether you’re OK with that in your presidential candidate is a subjective judgment, not an objective one.

There are other complications as well, Moberg said. Like the fact that some age-related deficits can be compensated for with a little help, depending on the demands of the job. While you wouldn’t want a forgetful surgeon, you might be able to tolerate it in your beloved family doctor — if she compensated by hiring a note-taker or relying more on electronic medical records.

And being an advanced age might help some presidents, since those years could come with skills that make up for a cognitive decline. The knowledge, political savvy and wisdom of a lifetime in office could be a decent tradeoff for a president who needs a little more help from aides to take notes.

Or … it might not be. That’s really the trouble, Appel told me. The tests can tell you how a person’s brain is functioning, but they can’t tell you how to value that score as a voter. (Or as a member of Congress who has to work with that president.) It’s a piece of objective data with a subjective interpretation. Ultimately, while both Appel and Moberg thought it made sense to run cognitive tests on members of high-stakes professions, like the presidency, Appel said the final judgement was really the election itself. “I mean,” he said, “there are probably [candidates] who everyone can agree clearly fall below the floor — but nobody is voting for them.”

FiveThirtyEight’s 2020 draft: Episode 2