You often hear the complaint that modern political campaigns are too nasty, which is often true. But it's also often true that they are too polite. So we're fortunate when someone violates the prevailing etiquette, forcing candidates and voters to confront matters we'd rather not but should.

One of those moments came the other day when Sen. John McCain visited a New Hampshire high school and took questions. One brave youngster asked him, as courteously as possible, if at 71, he might be too old for the job he's seeking. McCain scoffed, saying, "I work 24/7, I'm very active, I enjoy life," and bragging that he's always outcampaigned his opponents. He closed by joking, "Thanks for the question, you little jerk. You're drafted."

But the student raised an important question that many of his elders have been strangely unwilling to pursue. It may seem rude and even cruel to say that someone is simply too old to be entrusted with the presidency. But in McCain's case, by any sensible standard, it's also true.

If elected, he would be the oldest person ever to enter the office. Ronald Reagan was 69 when he was inaugurated, nearly two and a half years younger than McCain will be on Jan. 20, 2009. If he served two full terms, McCain would leave the White House at the age of 80.

Yes, he appears to be an active man who doesn't tire easily. You don't get to the top of the political heap by working bankers' hours. But to claim that because he's energetic at 71, he will suffer no slowdown in the next five years is like saying that because he's still breathing, he won't ever stop. A car could run fine for 200,000 miles, but if you're driving across Death Valley in July, that vehicle might not be the best choice.

The only certainty in life is that age catches up with all of us eventually. In the case of a man McCain's age, the odds are it will happen sooner rather than later. Each additional year increases the likelihood of physical infirmity and mental deterioration, not to mention death. A report last year by the Mayo Clinic found that one out of every 11 people it studied between the ages of 70 and 79 had some cognitive impairment, often a precursor to Alzheimer's disease.

McCain thinks he's the exception to the normal rules of aging because he has a 95-year-old mother who, by his account, is still sharp. What he neglects to mention is that he's already outlived his father and grandfather.

In any case, he's endured more wear and tear than the normal AARP member. As a Navy pilot during the Vietnam War, he broke both arms and a leg in a crash after his plane was shot down. He spent five and a half years being tortured, beaten and half-starved as a prisoner of war in North Vietnam. He's had surgery twice for melanoma, the deadliest form of skin cancer. All of those misfortunes exact a toll that may offset his hardy genes.

In other lines of work, everyone accepts that there is such a thing as too old. Some major corporations force chief executives and directors to step down at age 65 or so. Airline pilots have a mandatory limit of 60, which the Federal Aviation Administration has decided to raise to 65. Law firms often put partners out to pasture once they reach the golden years.

In those jobs, a fixed age limit makes less sense than it does for the one McCain wants. If a lawyer can no longer handle the work, after all, the firm can promptly cashier him or her. But the voters may never know if a president is growing befuddled by routine tasks -- or if a president, wearied by age, has simply lost the energy needed to perform well. And even if such facts became known, the public may not be able to force his removal.

That's not a big deal for a senator, who can't do much without 50 other people. But for the person occupying the most consequential office on Earth, it's an alarming prospect. John McCain has done a lot of things for his country. He could do one more service by acknowledging that the presidency is a job for a younger person.