In 2008, John McCain admitted he didn't use a computer, prompting negative ads by then-candidate Barack Obama saying he was "out of touch." In 1992, a 45-year-old Bill Clinton surprised the political world by picking Al Gore, 44, as his running mate, setting up a youthful duo to run against a 68-year-old George H.W. Bush.

And back in 1984, when Ronald Reagan was asked in a debate whether he had the stamina needed for the job, his famous quip -- "I am not going to exploit for political purposes my opponent's youth and inexperience" -- got plenty of laughter.

Age has long been a factor in presidential campaigns -- a litmus test for experience, a line of attack in political ads, a way of signaling to groups of voters about either new, fresh leadership or hard-earned veteran wisdom.

Yet in this election -- the oldest matchup between presumptive major party nominees in history -- age is not spoken about nearly as much. Donald Trump turned 70 last month, and would be the oldest president ever elected if he were to win in November. Hillary Clinton will turn 69 just before the election, and if she wins, would be the second oldest president to take office, after Reagan.

Yes, Trump has lashed out with attacks about Clinton lacking strength or stamina. But it hasn't seemed to stick, with other issues like the email controversy taking center stage, and there haven't been many others questioning the candidates' age. "There's no longer a bias about age and the presidency," says Douglas Brinkley, a historian at Rice University and author of several books about presidents. 

Indeed, a McClatchey-Marist Poll from last November -- when there were still plenty of younger candidates in the race -- showed that 71 percent of registered voters actually consider advanced age a benefit to the presidency, and that even among the youngest voters, 67 percent would have no problem supporting a candidate over 65. Meanwhile, the oldest candidate in this year's primary race -- Bernie Sanders, who is 74 -- has been a favorite among the youngest voters.

One obvious reason age isn't a theme of this campaign is that when both candidates are of a similar age, there's little contrast to make between the two. "It makes it moot," Brinkley says. He doesn't think it will be a focus of attacks by either candidate, "even if they attack each other on everything else." 

Beyond that, historians say age isn't getting much attention because of the highly abnormal nature of this year's campaign. "You’re dealing with such unusual candidates -- both of them -- that in some way it overcomes age," says Julian Zelizer, a professor of history and public affairs at Princeton University. Clinton is most identified with being the first female major party nominee, as well as a polarizing, well-known political figure, he says, while Trump is a truly atypical political outsider.

"The particularities of these candidates has made an issue that often does come up less relevant and less interesting," Zelizer says. "If it was, say, Dick Cheney running against maybe Joe Biden, you probably would hear more about age." 

He also says that whatever voters might think of either candidate, it's hard to believe either of them are lethargic or lack the energy to do the job. Clinton has built a career reputation as an indefatigable hard worker, while Trump appears to "have as much energy as someone who’s still in his 20s," Zelizer says, and someone who took pains during the primary campaign to "quickly turn Jeb Bush's 'low energy' against him. In a way, it was deflecting his age." 

Both candidates also represent broader issues that are tied to youthful characteristics, which helps to avert attention from their age. Trump, says Leonard Steinhorn, a professor of public communication and history at American University, represents a disruption to politics, a theme often tied to youthfulness. In addition, "some people would say he has adolescent qualities," Steinhorn says. 

Clinton, meanwhile, is the "quintessential baby boomer," he says, a generation so associated with youth and challenging norms that it makes her age less of an issue, even if she comes off now as a clear member of the political establishment. "The fact that both of them can lay claim to characteristics associated with youthfulness mitigates any negative associations with their chronological age."

There may also be a more permanent shift going on, one that has less to do with the characters of this election and more to do with shifts in American society or demographics.

With Americans living longer, making age a factor in the campaign is likely to turn off older voters. And as the ranks of aging baby boomers grow, don't expect to see ads that condescend around the issue of age, even in future elections when there might be more of a contrast between candidates' years. It could cost them substantial votes.

Meanwhile, in an American zeitgeist that has always valued youthfulness, attitudes toward retirement and old age are shifting rapidly in ways that could change how people view leaders, too. "The idea that 'with age, comes wisdom and experience' is being replaced by 'with age, vitality continues,' " Zelizer says. "There’s no need to discount people who are in their 60s and 70s."

Read also:

Like On Leadership? Follow us on Facebook and Twitter, and subscribe to our podcast on iTunes.