When John McCain showed up May 17 as a surprise guest on

Saturday Night Live

, he pitched himself this way:

"I ask you, what should we be looking for in our next president? Certainly, someone who is very, very old. . . . It's about being able to look your children in the eye - or, in my case, my children, grandchildren, great-grandchildren, great-great-grandchildren, and great-great-great-grandchildren, the youngest of whom are nearing retirement. . . . I have the courage, the wisdom, the experience, and, most importantly, the oldness necessary. The oldness it takes to protect America. . . ."

Translation: The McCain people are deathly afraid voters in November will balk at electing the oldest president in U.S. history.

Which is precisely why they booked the candidate on a hip show (message: "He's young at heart"), where he could perhaps defuse the age issue by bringing it up himself as a jest (thus hewing to the old political maxim, "Hang a lantern on your problems"). Actually, because the show is broadcast in high definition, a pitiless technology that exposes every facial flaw, no matter how minuscule, McCain did look very, very old, but his people couldn't do anything about that.

And they're probably right to be concerned. Amid all the talk about Barack Obama and Hillary Rodham Clinton breaking racial and gender barriers, it's easy to forget McCain faces a potential age barrier. We've never elected a first-time president in his 70s; this summer, McCain will be 72. And, assuming 46-year-old Obama is tapped to be his rival, never before have two candidates been so far apart in age.

Senior citizen status is hardly fatal in politics, of course. Ronald Reagan was 69 when he was elected in 1980, and Andrew Jackson was elected in 1828 at age 61 and reelected at 65, in an era when the average American lived to be 45. As I'll argue later, the most important determinant is not the age of the candidate, but the vitality of the candidate's message.

On the other hand, we do live in a culture obsessed with youth, and it's undeniable that, whereas racism and sexism are frowned upon, ageism is grist for the jokesters. Witness David Letterman on John McCain: "He's the kind of guy who picks up his TV remote when the phone rings. . . . He looks like the kind of guy who goes to the curb for the paper and locks himself out of the house."

Indeed, virtually every poll suggests that, at least with respect to the burdens of the presidency, Americans don't necessarily buy the notion that 70 is the new 60. The nonpartisan Pew Research Center has served up some startling figures: 50 percent of registered voters say they're less likely to support a candidate in his 70s. That's higher than the share of voters who would be less likely to support a Muslim (48 percent), or a gay (46 percent), or a Mormon (32 percent). That poll didn't mention McCain by name, but Pew did so in a separate survey - which found that 19 percent of Republicans deem him too old for the job.

No wonder the McCain camp is so touchy about his mileage. On May 8, when McCain painted Obama as the candidate most preferred by the terrorist group Hamas, Obama retaliated by suggesting that perhaps McCain had "lost his bearings," a remark that in turn prompted a furious McCain aide to paint Obama as a dirty dealer of the age card. (Republicans are accustomed to smearing Democrats as terrorist-friendly weaklings on national security and not suffering any verbal blowback.)

And, just the other day, the McCain people finally agreed - after 15 months of procrastination - to release the candidate's health records, which include details of his 2000 surgery for a serious melanoma. (McCain has weathered four melanomas, only one of which was serious, Stage II on a scale of four.) They were released Friday - the Friday before a holiday weekend, a strategy designed to draw minimal attention. Not only would McCain's people prefer his age not be discussed; they also would prefer that voters preparing to elect a president not dwell at length on cancer survival rates.

Indeed, McCain himself has mentioned the age factor. In March 2000, as his first candidacy was flaming out, he signaled he was reluctant to run again: "If I were 43 or 53, it might be different. But I'm 63, a pretty old geezer."

In the end, McCain's biological age in 2008 is far less important than his metaphorical age. That is really the crux of his challenge this fall. An older candidate can win if his message is perceived as new; Reagan won a landslide reelection in 1984, despite some qualms about his age, because his "Morning in America" image was perceived as fresher than Walter Mondale's old-style liberal politics.

Conversely, Bob Dole played old in 1996, not necessarily because he was grumpy and 73 and fell off a stage in front of the cameras, but because his message seemed stale in comparison with incumbent Bill Clinton's talk of building "a bridge to the 21st century." Clinton successfully framed that election as a choice between future and past, with Dole cast as the latter.

McCain's challenge is to escape Dole's fate. Potentially, the melanoma scar on the left side of his face is far less politically threatening than the scars left by the Bush era. Voters may not necessarily debate whether McCain seems too exhausted for the strenuous presidency - but they might focus on whether McCain is too closely affiliated with an administration that has left America exhausted by war and incompetence, and thus too rooted to the old ideas. Showing up on Saturday Night Live, and hobnobbing 10 times (so far) with the host on The Daily Show With Jon Stewart, may not prove to be enough.

Obama has indeed taken some subliminal swipes at McCain's age; his frequent lauding of McCain for his "lifetime of service" is the rhetorical equivalent of praising grandpa at the assisted-living facility. But Obama's broader intention is to capture the metaphor, framing himself as messenger of the future and McCain as embodiment of the past. The prevailing political climate might well buoy his efforts.

Contact Dick Polman at dpolman@phillynews.com. See his blog at http://go.philly.com/polman - and watch for excerpts in the daily Commentary Page.