Voters fill in their ballots as they vote in the midterm elections at a polling place in Westminster, Colorado, in 2014. Reuters

The first time Noel Diem voted for president, in 2008, she had just turned 18 and she was sure of her choice. The Reading, Pennsylvania, resident was planning to be a teacher, so she backed then-Sen. Barack Obama after reading about his education policies in her local newspaper.

A lot will have changed for Diem when she votes in her state's Democratic primary this spring. She's 25, and she ended up in marketing, not teaching. Now that she's shopping around for healthcare plans, access to affordable healthcare is her top concern. And she can't decide who to support: Vermont Sen. Bernie Sanders or former Secretary of State Hillary Clinton.

"My head and my heart say different things," Diem said, adding that she vacillates between the candidates often while trying to decide. She partially blames the fact that her priorities have expanded and evolved over the past eight years.

Her case is not unusual: The young voters who helped propel Obama to victory over Clinton for the Democratic nomination in 2008 aren't the same young voters flooding the polls for Sanders in Iowa and New Hampshire. Political experts say the demographics and opinions of the millennial generation have shifted over the past eight years. With the United States less militarily engaged in Iraq and Afghanistan than in previous years, young people struggling to recover from the Great Recession (2007-09) have increasingly identified the economy as their No. 1 issue. Even first-time voters without mortgages and kids to worry about are looking at their finances.

"When you ask, 'Is the economy a problem?' a good majority say yes," said Kei Kawashima-Ginsberg, director of the Center for Information and Research on Civic Learning and Engagement at Tufts University in Medford, Massachusetts.

The millennial generation usually includes people born between the 1980s and 2000s, though some organizations move the endpoints inward to start at 1982 or end at 1996. In most definitions, "millennials" refers to the Americans who today are 18- to 34-years-old. In other words, the oldest millennials in 2008 were 26. Between then and now, a middle cohort has come of age, and there's a small subsection that still can't vote today.

In November 2008, more than half of millennials cast ballots in the general election. But months before that, they were also decisive in choosing the Democratic nominee. On Super Tuesday 2008, young voters chose Obama by 16 points.

Obama had vowed to end the war in Iraq, which 37 percent of young Americans had identified as their top concern in a fall 2007 poll from the Harvard University Institute of Politics. Only 5 percent of respondents considered the economy the most important national issue, but this changed as time wore on.

By summer 2008, 39 percent of respondents were telling Harvard the economy was the most important national issue while 15 percent said the Iraq War. About half of respondents to a 2008 USA Today/Gallup poll said "dealing with the economic crisis" was an "extremely important" job for the next president, while about a third said the same for "dealing with Iraq and Afghanistan."

The intervening years saw more than just the rise of selfies and a national obsession with the Kardashians. While concern about the economy has remained high, millennials' interest in foreign policy seems to have fallen. A fall 2015 Harvard poll found only 9 percent identified foreign policy as their No. 1 worry. A January 2016 survey by USA Today found that 35 percent of youth named "economy/jobs/employment/minimum wage/paid leave" as one of the most important issues, followed by "education/college affordability/student debt" and "foreign policy/the Middle East/terrorism/homeland security."

Kawashima-Ginsberg attributed the shift to a change in demographics and life experiences. "The young people that experienced that amazing election in 2008 that changed the game and excited young people are no longer really under 30 anymore," she said.

Older millennials who were hit hard during the recession tend to support strong Social Security policies. But the younger ones have struggled with slow recovery. Take wages, for example: In 2008, median annual earnings of 25- to 34-year-olds working full time totaled $38,940. As of 2013, they added up to $40,000.

"When the recession officially ended in 2010, jobs were supposed to be getting more and more available, but that wasn't really the case for the youngest people in the workforce," Kawashima-Ginsberg said. "Now in their mid-20s, they're showing slightly different orientations in how they identify themselves in the parties."

A 2014 Pew Research Center study found only 39 percent of people said their finances were in "excellent" or "good" shape. First-time voters entering college or beginning to take on other adult costs may also be "feeling the squeeze," Kawashima-Ginsberg said, adding that different groups of youth have different specific concerns under the economy umbrella. For example, white youth are worried about the federal deficit, while minority youth often mention unemployment. They're going to choose the candidate they think can best address these problems.

Mike Hais, co-author of "Millennial Momentum: How a New Generation is Remaking America," said he doesn't think today's young voters are necessarily less liberal than before. Economic issues have simply stayed at the top of young voters' lists because their formative years occurred during the Great Recession. Instead of focusing on national security, "they are trying to get their lives together," he said.

Although youth unemployment has been declining since 2010, 26 percent of young adults were living in their parents' homes as of last January. About 70 percent of college graduates in 2014 had student loan debt. They may think Sanders, who has built his campaign around busting Wall Street billionaires and making tuition free, would help them most.

"When they're in college, it's more like, 'I've got to get my degree. I've got to get ready.' ... but as they age I think these realities hit them in many ways," Hais said. "Economics is a huge concern for them."

This trend has coincided with an anti-authoritarian movement among young people, said Emily Ekins, a research fellow at the libertarian Cato Institute in Washington, D.C. Voters, and particularly millennials, have begun to move away from order and conformity in favor of authenticity and individual liberty. That's one of the themes Sanders promotes most.

"He just seems so honest," Ekins said.

But overall, in terms of social issues the qualities of the youth vote today aren't that different than they were years ago, said Elizabeth Matto, director of the Youth Political Participation Program at Rutgers University in New Brunswick, New Jersey. "The real question," she said, "is whether or not the campaigns and candidates have changed."

The millennial generation tends to have an activist streak, Matto said, pointing to Twitter campaigns like #BlackLivesMatter and #BringBackOurGirls, the effort to draw attention to Nigerian girls abducted by the militant group Boko Haram. Candidates who capitalize on that desire for social justice in their messaging could draw young voters to their side. This strategy could play out especially well because millennials are also becoming more diverse: In 2008, young voters were 60 percent white. Eight years later, that's dropped to 56 percent.

That's an issue to which Chelsea Marks, a 26-year-old who works in higher education in Austin, Texas, is paying attention. Marks said she's undecided as to which Democratic candidate she wants to support, and she's waiting for Clinton or Sanders to take a bold stance on racial inequality.

Marks calls herself a "social voter," but she also falls in line with her peers fiscally. She said she's trying to save money to buy a house with her husband, so she's more concerned now with the economy than she was in 2008.

"The core issues I care about the most are the same," Marks said. But, she added, "I know a little bit more about the world, for sure."