Eight years ago, when John McCain was preparing to drop out of the race for the Republic nomination, he predicted that he probably wouldn't run again: "If I were 43 or 53, it might be different," he said. "But I'm 63, a pretty old geezer." So what does that make the soon-to-be-72 McCain? "Sensitive," is one answer. When Barack Obama said last month that McCain is "losing his bearings as he pursues this nomination," McCain aide Mark Salter fired off a memo that called Obama's words a "not-particularly-clever way of raising John McCain's age as an issue." Salter said the "bearings" comment was just "typical of the Obama style of campaigning."

So I take it that John McCain's age, like Hillary Clinton's sex and Barack Obama's skin color, is not supposed to be raised as an issue. Indeed, the campaign's "three -isms" - racism, sexism and ageism - often get mentioned in the same breath. A piece in the New York Times recently bore the mystifying title "Age Becomes the New Race and Gender," and explained that this year's campaign "has provided an extended test of attitudes" towards all three. Not to be outdone, this typically silly New York Sun column contends that "Race and age are the great subterranean themes … at play in the presidential election and are at least as potent as the differences between the rival candidates over issues such as health care and free trade." Indeed, the author suggests that McCain might have it worse, since race "is strictly off limits while a person's age, it seems, is fair game for humor."

But there's a reason why age is fair game for humour: it's also fair game for voters. Age isn't the "new" race and gender. It isn't even the old race and gender. McCain's age is a legitimate issue in this election. Unlike race and gender, age has necessary implications for anyone's ability to perform the job.

Why is age different? The main reason why age matters is that the job of president is difficult to perform while dead. The average life expectancy for an American male is just shy of 75 years, and while the McCain campaign is fond of rolling out the senator's 96-year old mother - as in this particularly shameless example - they are substantially less eager to mention that McCain's father died at 70 and grandfather died at 61. And there are a variety of maladies that might grab you on the way to the grave: illnesses, memory loss, lack of energy - plenty of health problems to go around. (For some speculation about what ageing might mean for McCain's brain, see this Slate article.)

To be sure, you wouldn't want the candidates' age to be the dispositive factor governing your choice for president. It would be strange and foolish to summarily reject a candidate after he passed a certain age, in the same way that it would be strange to prejudicially reject, say, blind candidates. Responsible voters should judge candidates as individuals, not as vast aggregations of class membership. But this doesn't mean qualities like age or vision don't have real effects on one's ability to perform a job, and can't serve as shortcuts for democratic decision making. Indeed, the government uses these shortcuts all the time: federal law requires that airline pilots retire by age 65, despite the fact that there are almost certainly some pilots who could continue flying until the ripe age of 70. Maybe even 72.

But if grouping race and age in the same category makes no logical sense, it does serve a very real argumentative purpose. Actually, two purposes. First, mentioning McCain's age alongside Obama's race is an attempt to silence discussion of the elder senator's health, and thus disarm a legitimate campaign issue. (The goal of Salter's memo is to make you feel like a regular Roger Taney if your words bear even the faintest concern for McCain's chronological challenges.) Second, conflating age and race is an attempt to silence claims from Obama supporters that their candidate is at a unique disadvantage because of his race. According to the McCain campaign, we're all victims here - or, at the very least, there's plenty of room under the mantle of victimhood.

I'm not buying it: Obama's race is a real disadvantage, and John McCain's age is both a real and reasonable issue in this campaign. But maybe I'll think differently in a couple of decades. Or maybe I'll just forget I've written this.