Over and over, Sen. Bernie Sanders (I-Vt.) insisted he had the formula for winning in November. He would bring working-class whites back to the Democratic Party and get young voters to the polls, both challenges that helped Donald Trump win the White House in 2016. He insisted he could drive turnout — but turnout surges in the primaries seemed to favor former vice president Joe Biden.

Myriad reasons account for this, including Biden’s benefit from the consolidation of the Democratic field. But one clear reason is younger voters, while supportive of Sanders, couldn’t offset older voters’ broad preferences for Biden.

Turnout rates correlate nicely with age. In 2014, young Californians voting for the first time turned out at higher rates than those in their early 20s. As the age of voters increased from then on, so did turnout rates — until rates dropped off again among the oldest voters.

Across the country, turnout rates for Americans 60 and older are significantly higher than for people younger than 30. In 2016, the oldest voters turned out at a rate of 71 percent, according to Census Bureau data analyzed by the United States Elections Project. That was nearly 30 points higher than the rate for those under 30. In 2018, the highest-turnout midterm election in decades, younger voters turned out at rates 30 points lower than the oldest voters.

Why? Voting is a habit that one learns over time. Voting also tends to correlate to behaviors and conditions favoring older voters.

After the 2016 election, the most common response from Americans who didn’t vote was they didn’t like the candidates, according to analysis from Pew Research Center.

Importantly, the frequency of that response was consistent among age groups: a quarter of each generational group under age 70 cited not liking the candidates as their main reason for not voting. (For those 70 and up, only a fifth identified dislike of the candidates as their main reason.)

Notice the other reasons that were offered.

Many people said they were too busy to vote on Election Day, an typical obstacle for people with multiple jobs, odd hours or entry-level positions — jobs younger people might be more likely to hold. Other respondents said they couldn’t get to polling places, perhaps lacking a car or because their polling place was far away.

It’s much easier to know where to vote if you’ve lived in the same place for 20 years and your polling place never changes. It’s much easier to vote the less you move, which requires re-registration every time. It’s much easier to remember to vote if you vote in every election and get barraged with campaign voting appeals. It’s much easier to remember to vote if you are the boss and can vote whenever you want or if you’re a retiree with more than enough time to vote. It’s much easier to vote if you can walk or drive to your polling place instead of having to catch a bus.

It’s also much easier to vote if the powers that be want you to vote. Younger voters tend to vote Democratic, which is a disincentive for Republican legislators. Last October, the New York Times documented examples of efforts to tamp down on voting among younger people, including a law in New Hampshire reducing the ability of college students to vote in the state and new efforts elsewhere to limit polling places at colleges and universities.

Young people voting heavily Democratic overlaps with younger people more likely than older Americans to be nonwhite. That means efforts to suppress the nonwhite vote can ensnare more younger people as well.

The Center for Information and Research on Civic Learning and Engagement at Tufts University, looking at why younger people hadn’t voted, broke out one set of data by race. Difficulty finding polling places, securing transportation and having an ID were cited more likely as reasons for not voting among nonwhite young people than whites.

A standard response to low turnout rates among young Americans is a hand-wavey, just go vote, as though it’s simply a function of will. It isn’t. Sanders’s campaign didn’t presume it was, putting significant resources into making it easier for young people to get to the polls.

It turned out to be a harder problem than the candidate’s rhetoric suggested.