When it comes to personal characteristics and diversity, the 2008 presidential race has covered a lot of ground. Are Americans prepared to embrace an African-American candidate (Obama)? A woman (Clinton)? A Mormon (Romney)? A Latino (Richardson)? A thrice-married serial adulterer (Giuliani)?

But the one question that no one seems anxious to talk about is the fact that John McCain, at age 72, would be the oldest person ever elected president. There’s apparently some public discomfort over this, but it’s ground that few are prepared to tread.

In his big foreign policy speech the other day in Los Angeles, McCain began his remarks with a personal anecdote:

“When I was five years old, a car pulled up in front of our house in New London, Connecticut, and a Navy officer rolled down the window, and shouted at my father that the Japanese had bombed Pearl Harbor. My father immediately left for the submarine base where he was stationed. I rarely saw him again for four years.”

Now, I suspect the story was intended to remind the audience about the proud military history in McCain’s family, but there were probably more than a few people who heard the anecdote and thought, “Wait, McCain was already five in 1941?”

I mention this in part because the new NBC/WSJ poll (pdf) asked respondents if they think Americans are prepared to elected an otherwise qualified candidate, who happens to be have certain characteristics. 72% of Americans, for example, said the country is prepared to elect an African-American president, and 71% said we’re also ready for a woman president.

But when asked about a candidate over the age of 70, the number dropped to 61%.

This isn’t a new problem, which is all the more reason it’s surprising this is barely a blip on the political world’s radar.

I half-expected the age issue to be a bigger deal. Way back in February 2007, an WaPo/ABC poll asked Americans: “I’m going to read a few attributes that might be found in a candidate for president. Please tell me if each would make you more likely to vote for that candidate for president, or less likely to vote for that candidate, or if it wouldn’t matter.” When the attributes mentioned race, gender, religion, and marital status, poll respondents generally didn’t care at all. When the poll mentioned a 72-year-old candidate, 58% said they would be “less likely” to vote for such a candidate — more than the totals for a woman, African American, and/or Mormon combined.

Around the same time, a USAT/Gallup showed that 42% of voters said they wouldn’t support an otherwise qualified 72-year-old candidate.

But that was well over a year ago, before the race began in earnest. Have the numbers changed? Not much.

A CBS/NYT poll last month asked American what the best age is for a president. A majority (55%) preferred someone in their 50s, while a president in his or her 40s was second (with 26%). How many preferred someone in their 70s? Less than one percent.

Earlier this month, the WaPo polled on this again, and found that Americans still aren’t on board with electing the oldest president in history. Poll respondents were told that Hillary Clinton would be the first female president. 20% said that makes them more enthusiastic about her candidacy, while 11% said less. They were told that Barack Obama would be the first African-American president. 16% said that makes them more enthusiastic about his candidacy, while 11% said less. And when told about McCain’s age, 4% said that makes them more enthusiastic about his candidacy, while 27% said less.

Here’s the catch: Americans may not like the idea of a 72-year-old candidate, but no one has any idea how to take advantage of this. What are Dems supposed to do, tell elderly jokes? That’s clearly not going to happen.

Voters may not be comfortable with McCain’s age, but figuring out what to do about this may be one of the more complicated questions facing Dems in the general election.