This isn't to say that age should be the deciding factor in a campaign, or that voters should necessarily rule out older candidates. But age is an undeniable factor when assessing those who aspire to the world's most demanding job.

After the 2008 election, many Republicans concluded that it had been a mistake to nominate someone as old as John McCain, then 72. Barack Obama was 25 years his junior, becoming the nation's fifth youngest chief executive.

Republicans faced the same issue back in 1996 when 73-year-old Bob Dole lost to Bill Clinton, who was only 50 at the time. Both Dole and McCain looked their ages as the long campaigns wore on, especially when matched against younger and more vigorous opponents.

McCain had a list of quips ready about his age, often joking that he could hide his own Easter eggs. Reagan, 73 when he campaigned for a second term, also used humor to his advantage with a memorable zinger in a 1984 debate with former vice president Walter Mondale. "I will not make age an issue of this campaign," said Reagan. "I am not going to exploit for political purposes my opponent's youth and inexperience."

Unlike other campaign considerations, age is a fuzzy issue. Most reasonable voters would insist, for example, that gender is not relevant in considering Michele Bachmann's candidacy, that race had nothing to do with Herman Cain's qualifications, and that religion should not be a factor in evaluating Mitt Romney and Jon Huntsman. Yet, though we occasionally hear of "age discrimination" in the workplace, age does become relevant when they seek the nation's highest post, as our two oldest presidents proved.

Harrison took ill and died a month after inauguration. And Reagan, according to his son Ron and many who worked closely with the septuagenarian chief executive, suffered early effects of Alzheimer's while in office.

Remember when Herman Cain, who turns 66 next week, fumbled when asked about Libya? Something seemed wrong beyond the fact that he is not well versed in foreign affairs. When he said, "Got all of this stuff twirling around in my head," it seemed to some that he was having a so-called senior moment.

Older candidates stress that it's their physical condition rather than age that should count. Besides, Americans are living longer — much longer, in fact, than in Harrison's day — with the average life expectancy now 78. The Census Bureau reports that 1.9 million Americans are now over the age of 90.

But even with greater predictable longevity, the older GOP front-runners are seeking four-year terms at ages when most Americans retire or at least begin to cut back. During the 2008 campaign, 70% of voters surveyed by CBS News said the best age for a president was under 60.

If one of the current GOP front-runners wins next November, he will still be in office five years from now. Romney will be 69, Gingrich 73 and Paul 81. Those ages give new meaning to Grand Old Party.

(c) Peter Funt. This column originally appeared in USA Today.