Youth voter turnout so far in the Democratic primaries is either flat or declining compared with the 2016 primaries.

Why it matters: Sen. Bernie Sanders has based much of his strategy on the hope that he could turn out large numbers of young voters. The apparent decline is bad news for him, but it will also make it more difficult for future-focused issues like climate change to gain political traction.

Context: According to the Harvard Institute of Politics, while raw turnout is up in all 12 of the states with competitive elections, the youth vote has only risen in four states, and is flat in two other states.

Of the 14 states that held primaries on Super Tuesday, participation by voters younger than 30 didn't exceed 20% in any state, according to exit poll analyses.

Young voters have always turned out at lower percentages than their older counterparts. But in Sanders, young Americans had a candidate who is explicitly pushing for their support — yet so far it hasn't seemed to matter.

Yes, but: Some experts suggest that the picture isn't as bad as it looks.

The Center for Information & Research on Civic Learning and Engagement at Tufts University told the Washington Post that youth turnout so far is mostly higher compared with 2012.

"It's possible this cohort is less ideological than we thought, and is willing to show up to combat Trump more than they're willing to show up to support a Democratic socialist agenda," said Charlotte Alter, a national correspondent for Time and the author of the book "The Ones We've Been Waiting For."

The bottom line: Young people have the greatest stake in the future. But they can't shape it if they don't vote.

Go deeper: 2020's new voters