It was probably not wise for the 64-year-old Brit Hume to describe the 71-year-old John McCain as having a "senior moment." A blip would have been better. Or a gaffe. Or even a dent in the candidate's "experience" armor.

But when the traveling senator confused Shiites and Sunnis, when he conflated al-Qaida with all extremists, the "senior moment" phrase uttered by the Fox newsman got stuck to the story of The Man Who Would Be the Oldest President in American History.

Age? Ageism? Or realism? We've been holding a heated conversation about race and gender all season. But age has been relegated to a late-night laugh line by the likes of David Letterman, 60, who described McCain as "the kind of guy who picks up his TV remote when the phone rings."

The candidate, no slouch in the self-deprecation business, refers to himself as "old as dirt," although he travels with his 96-year-old mother as a genetic ambassador. And when a New Hampshire high schooler asked McCain whether he might die in office or get Alzheimer's, he answered, "Thank you for the question, you little jerk."

Nevertheless, it's worth assessing this senior's moment in politics. The polls suggest that Americans are more reluctant to vote for a 70-year-old than for an African-American or a woman. Before you attribute this to prejudice, remember that only 24 percent of Americans under 35 think McCain is too old while 40 percent of those over 65 believe it. Do they know something we should know about a man who would be 72 on Inauguration Day and 80 at the end of two terms?

The cheery cliche of the moment is that 70 is the new 60. In fact, mental fitness has increased along with physical fitness. But at the same time, a new study shows that one out of every three Americans over 70 has some cognitive decline.

I'm willing to bet that McCain is in the lucky two-thirds of this population. And senior moments are not just for seniors. Did Hillary have a middle-aged moment about the sniper attack in Bosnia that never was? Did Barack have a junior moment when he wrote about reading a Life magazine article on a man who tried to lighten his black skin? An article that never ran.

But we have grown to expect a thorough health report on candidates. We knew about John Kerry's prostate cancer and Joe Biden's brain aneurysms. We know about McCain's war injuries and his melanoma, his cholesterol and his allergies. We expect full assessments from every doctor except, well, neurologists. If airline pilots, some judges, and people in other occupations are subject to cognitive tests, why not presidential candidates?

The subject is as uncomfortable as talking to an aging parent about giving up the car keys. Even the feelings among the experts on the elderly are mixed. On one hand, Laura Carstensen who heads Stanford's Center on Longevity, offers the good news that as people get older, their knowledge generally increases as does their ability to regulate emotions. Yet as a voter, she says, "I see better reason to know about someone's cognitive health than medical health."

So too, Robert Butler, the man who coined the word "ageism," says we should think about individual function, not age. But he adds, "We do want to be sensible when entrusting the leadership of our country that they're mentally as well as physically healthy."

Even the author of that study on the high rate of cognitive impairment talks about a "gray area." Duke University's Brenda Plassman warns that we can diagnose cancer or diabetes with great certainty, but "there's no real biomarker for cognitive decline." Nevertheless, isn't there information citizens want to have as politicians get their senior moment in the sun? At 60? 70? 80?

I can name many wise elder statesmen from Winston Churchill, prime minister at 80, to Nelson Mandela, who retired at 81. Yet my memory is still good enough to conjure up Ronald Reagan, whose Alzheimer's disease may well have begun while in office.

Information won't always make decisions easier. How would we balance the incline of wisdom against the decline of, say, memory? How do we test stress? And if we ask for cognitive tests, what's next? Genetic tests? But despite these limits, such information matters. More than a presidential cholesterol test.

I sincerely hope that 70 will be the new 50 before I get there. But for the moment, my favorite line belongs to former Israeli Prime Minister Golda Meier. "Being 70 is not a sin," said this septuagenarian. "It's not a joke either."

Ellen Goodman is a columnist for The Boston Globe. Reach her at ellengoodman@globe.com.