Read: Is something neurologically wrong with Donald Trump?

Joe Biden stays focused on single topics and ideas much more consistently than Trump, but he is often slower to find words than he was in the past. He veers sometimes into nonsensicality. In September, in response to a question about racism, he wandered into, “Play the radio, make sure the television—the, excuse me—make sure you have the record player on at night … make sure the kids hear words.” After Bernie Sanders’s heart attack last month, news reports also brought concerns about age to the fore. While Warren seems to have been spared from such speculation, she would still be the oldest person ever elected to a first term. Ronald Reagan was reelected at 73, and the end of his second term was, by some accounts, marked by the early linguistic signs of the Alzheimer’s disease that he would announce five years later.

Concerns about politicians’ ages are not limited to presidential candidates. The average senator is 62. Mitch McConnell, the 77-year-old Senate majority leader, is already past the U.S.-male life expectancy. If the country were to deem, say, the Social Security retirement age of 66 as the mandatory cutoff for a career in politics, it would amount to a total overhaul of government. It would eliminate not just the top four presidential candidates, but much of Congress.

At a conceptual level, any hard cutoff introduces a host of problems. Beyond meriting charges of age discrimination, age cutoffs are difficult to defend on practical grounds. An age minimum might rule out, say, an illiterate 4-year-old, but it’s at least hypothetically possible that a 110-year-old might be perfectly able.

At the same time, for all the promise of modern science, it seems unlikely that the best approach to ensuring candidates’ abilities to execute the duties of the office is to rely on them to tell us they are fine. Bernie Sanders has insisted he is running a “vigorous campaign.” (He has invoked vigor often.) Joe Biden has reminded crowds that with his age comes wisdom. Elizabeth Warren has vowed that she would “outwork and outlast” her opponents. Donald Trump said in April, “I’m so young. I can’t believe it, I’m the youngest person.”

Beyond such first-person reassurances of health, the process for outside assessment is opaque, optional, and arbitrary. It does not need to be.

The concept of “fitness for office” dates to the assassination of John F. Kennedy. The transfer of power to Lyndon Johnson raised a hypothetical question: What if Kennedy had survived, but in an impaired state? What if a bullet had grazed his frontal lobes and he was only briefly hospitalized, but thereafter he seemed unable to hold a train of thought? What if he started making bizarre comments and took to denying the existence of the moon? Such gray areas were the basis for the ratification of the Twenty-Fifth Amendment, in 1967. It created a way of removing a person who became unfit to serve.