Is there such a thing as 'too old' to run for president?

President Donald Trump turned 73 on Friday. Two of his leading challengers, Joe Biden and Bernie Sanders, are 76 and 77, respectively. A third, Elizabeth Warren, turns 70 on June 22. Two long-shot presidential contenders would spend their 70th birthdays in office. A third will be 90 before Election Day.

"I just feel like a young man," Trump told reporters on the White House lawn in April. "I'm so young. I can't believe it. I'm the youngest person. I am a young, vibrant man."

He was less charitable toward Biden, who is three years his senior, and his other rivals: "I would never say anyone is too old, but I know they're all making me look very young, both in terms of age and I think in terms of energy."

Because I am a geriatrician, friends and acquaintances often ask me what I think about all the older people running for president in 2020. It usually begins like this: "I don't mean to sound ageist, but …" Is it safe? they wonder. Is it sensible? Does it matter?

The answers do not depend on a number. The criteria for running for or being president shouldn't vary with age, and age doesn't tell you what you need to know to determine a person's intellectual, emotional, physical or experiential fitness for office, whether that person is 38 or 90.

That so many older candidates vie for president shouldn't surprise us. Across states and industries, unprecedented numbers of people in their 70s today remain in or rejoin the American workforce. But the picture is confusing.

Americans over 65 have become the fastest-growing worker group in the United States; at the same time, conversations about early retirement, workplace age discrimination and a dominant industry that defines 30 as old abound.

Reasons not to retire at the traditional age of one's early- to mid-60s are many. Pensions are not what they were during what will most likely be regarded as the (historically exceptional) golden age of retirement in the 20th century. This means that people often delay retirement to maximize their retirement benefits, and many also keep working at least part time because they need the money to maintain their standard of living or keep themselves out of poverty.

Economics offer just one motivation for work in old age. Equally important are the benefits to health and well-being.

Not working often leads to loneliness and social isolation, which have a marked adverse impact on health; loneliness is as lethal as smoking 15 cigarettes a day, and social isolation increases risks of disease, hospitalization and death. A recent study suggested that early retirement kills men.

Importantly, the benefits of continued work include not just the absence of negatives but the presence of positives: increases in sense of purpose, life satisfaction, physical health and self-worth.

Those running for president aren't immune to any of these benefits from working, and they may contribute to the reasons septuagenarians want to run. But they don't answer the question of whether they should run and in what ways, if any, we should consider their age in assessing their fitness for office.

Telling examples show how statistics can be cherry-picked to support different agendas in answering those questions. On the one hand, older people are more likely than younger people to get sick, suffer adverse consequences from their medical treatment, or die. That means a septuagenarian president is at higher risk for missing work or dying in office than one in her or his 50s. On the other hand, most healthy 70-somethings don't die. Women in their 70s can expect to live another 17 years on average — meaning half of them will live longer than that — and men can expect an average of 15 additional years.

Just as adulthood differs from childhood, so too does elderhood differ from adulthood. As we move into our 60s and beyond, physical and cognitive ability become better predictors of survival than age, though age does matter, particularly in the later substages of elderhood. In other words, there is no set age at which a person becomes too old to run for president (or teach, or stock shelves, or drive for a ride-share company, or become an entrepreneur).

But absent a premature death, the vast majority of us will reach a point at which doing those things is unwise, unpleasant or impossible.

When that time comes is hugely variable. At no time in life are people as varied in their abilities, health and inclinations as in old age. Some people experience accelerated aging as a result of chronic disease, mental illness, poverty, incarceration or other medical and social circumstances. Others thrive into advanced old age.

Our society has long had age markers for behaviors that have a personal and social impact. These include going to school, driving a car, drinking alcohol and collecting Social Security. But just as we apply different (ideally equivalent and just) standards to children and adults, as well as to their subgroups, so too we should develop unique and fair standards for elderhood and its subgroups.

Discrimination flourishes when we don't discuss important social issues openly, honestly and with more attention to facts than biases.

There is probably a point at which a person is too old to run for president, but at what age that happens will vary by individual. For me, 100 is too old, and 90 probably is, too. The 80s are the ironically titled gray area. I know many people in their 80s who can work long hours. But being president isn't just any job, and I also know the risk for grave and fatal health events is high for an octogenarian.

This is a great moment in history for people in elderhood or headed that way — in other words, for most of us. Suddenly there are older people in the media, on the street, in our workplaces. There is an international movement of women showing their gray or white hair, me included. Having gray or white hair is bad only if being old is bad, and if being old is bad, we are all in trouble.

For our older presidential candidates, the bottom line is multifaceted, like elderhood itself: They are at higher risk for health issues, yes, but chances are that if they are healthy now, they will be just fine four years from now, too.

All candidates come with risks. The fairest and most sensible approach is to judge them on the basis of their qualifications and policies, not their age.

Louise Aronson is a geriatrician and professor of medicine at the University of California, San Francisco. She is the author of "Elderhood: Redefining Aging, Transforming Medicine, Reimagining Life." She wrote this for the Washington Post.