Illinois Sen. Barack Obama and New York Sen. Hillary Clinton have divided the Democratic Party by race, income and education, but there is no demographic indicator that sorts the Democratic vote as starkly as age.

If you voted in one of the Democratic primaries or caucuses, your age probably determined your vote: The older you are, the more likely you were to vote for Clinton, and the younger you are, the more likely you were to vote for Obama.

Part of this divide is easily explained, since Obama is younger, 46, and Clinton is 60. But, Obama has a particular appeal to young people such as Zahir Rahman, a sophomore at Wake Forest University in North Carolina.

Identifying with Obama

"He's hip to our culture and seems to know where younger people are coming from," Rahman says. Then he explains Obama's appeal as "someone who's new, offers this idea of hope, of change, which really isn't attributed to either party or any of the other candidates who are running."

But that's not the only reason young people vote for Obama, says Cliff Zukin, a professor of political science at Rutgers University. Unlike their baby boomer parents, voters under 30 are incredibly tolerant.

"We have a woman candidate running against a black candidate for president," Zukin says. "If you had advanced that idea 25 years ago, people would have said that can't happen. And the young people today are so tolerant that they don't even think of that as an issue."

There have been "youth candidates" before in the Democratic Party, including Eugene McCarthy, George McGovern and Gary Hart. But this year, young voters are doing more than just getting excited about a candidate. They are actually turning out to vote.

Young Lean Democratic

In primaries and caucuses all over the country, voters under 30 have doubled and, in some places, tripled their turnout participation. Traditionally young voters have very low turnout, but now their participation nationally is reaching their share of the population. And the young voters who are turning out largely are Democrats, Zukin says.

"If young people come into politics and identify as Democrats, there's going to cause a sea change in politics," he says. "What we've seen with George W. Bush is that he has made Democrats out of young people the same way Ronald Reagan made Republicans out of young people."

People tend to form their partisan preferences in their 20s and stick with them, so this trend could be the beginning of a long-term political realignment. In the short term, if Obama is the nominee, age will be a big issue.

Underscoring McCain's Age

Obama is already teeing up a classic generational challenge to the presumptive Republican nominee, Arizona Sen. John McCain. On the campaign trail, Obama has said, "Now, I believe John McCain is a good man and a genuine American hero, and we honor his half-century of service to this nation."

The subtext of Obama's statement is that a half-century of service is a long time. Obama's message is simple: McCain is old. If he is elected, McCain will be the oldest president ever.

Sometimes McCain makes a joke out of his age.

At a Q-and-A with a group of newspaper editors, he was asked if he worried that voters might reject him because he will turn 72 by January 2009. McCain responded by pretending to nod off in his chair, to which the audience roared with laughter.

There will be a lot more talk about age, both funny and serious, if the general election match-up turns out to represent the biggest age difference in the history of American presidential campaigns.