Emily Collinson, 21, a senior studying international relations at American University in Washington, likes that Bernie Sanders is old enough to have participated in the civil rights movement in the 1960s. (Astrid Riecken/For The Washington Post)

At 74, Bernie Sanders — he of the mussed hair, frumpy clothes and grouchy demeanor — might not seem an obvious choice as a shining political knight for the millennial generation. America, after all, is the birthplace of the 1960s mantra “Don’t trust anyone over 30.” Even now, many Americans consider old people uncool at best — or even worse, irrelevant.

Yet for millennials — the generation roughly in their late teens to mid-30s — that attitude is so yesterday.

Across the country, young voters have shown an overwhelming preference for Sanders, helping to narrow significantly the gap between him and Hillary Clinton in the Democratic primary race and in one case to hand him a decisive victory. In exit polls in Iowa and New Hampshire, and in entrance polls in Nevada, more than 80 percent of 18- to 29-year-olds supported Sanders, who if he won would be the oldest person ever to take office as president.

To a generation hobbled by student debt and low earning power, Sanders’s talk of free education and redistribution of wealth has struck a chord. And his age? It’s not only not a deterrent, it is an attraction, imbuing him with wisdom and perspective and connecting him to historical events that millennials admire.

“He’s the grandfather that gets it,” said Dash Radosti, 20, who is studying public policy and finance at American University in Washington. “And he doesn’t try to hide what he is. He’s a very authentic person; that especially resonates with young people.”

Supporters cheer for Sen. Bernie Sanders during a rally at the Scope arena in February in Norfolk, Va on Feb. 23. (Jonathan Newton/The Washington Post)

For a generation that has embraced homemade everything — from bicycles to furniture to cured meat — authenticity is key.

“Young adults are looking for the #nofilter approach,” said Christine Whelan, a professor of consumer science at the University of Wisconsin at Madison, referring to Instagram photos that haven’t been touched up. “They want to know that it’s real.”

At the same time, they are angry about soaring housing and education costs and limited economic prospects. If Barack Obama in 2008 was for them a symbol of hope, Sanders is a symbol of their disillusionment.

“He’s grumpy, and they’re grumpy, too,” Whelan said. “We would think it’s odd that they would rally around a grumpy old man, but maybe it’s not; maybe that’s their form of counterculture statement.”

In fact, many say Sanders’s age gives him an edge.

“Who doesn’t love the grumpy grandfather that’s been on the right side of the fight for decades?” said Bennett Noonan, 30, a tech-support worker in Marion, Iowa. “His age definitely lends to a certain endearing persona.”

As social media has encouraged young people to become politically active, the idea of electing someone who once walked alongside the Rev. Martin Luther King Jr. also holds a certain romance.

“He’s been on the forefront of issues like the civil rights movement,” said 21-year-old Emily Collinson, an international relations major at American University. “Students are rallying around social justice now, and he’s really the embodiment of it, and while we’re new to that movement, he’s been in it for a while.”

After all, from where they sit, electing younger leaders hasn’t gone so well.

“We’ve been sort of accustomed to these candidates in their 40s, but now we’re going the total opposite way; we’re going old,” said Tyler Mackie, 31, a fashion and Internet entrepreneur in St. Louis. “Student debt’s climbing ridiculously high, social networks are not in place, there is no such thing as a corporate ladder anymore; it’s more like a corporate zipline. It isn’t working, and if it isn’t working, you try something else.”

One thing that works for Sanders is that he defies conventional ideas of how a candidate looks and acts. He has made no attempt to spruce up his wardrobe or tone down his old-school Brooklyn accent; to young supporters, that makes him more trustworthy.

“I’ve seen a lot of memes on the Internet where you get that crazy psycho guy who warns of impending doom, but in fact that guy turns out to be on the money when everyone’s discounted him,” Mackie said. “I think a lot of people in my generation look back on ‘Back to the Future’ ” — in which a tousled and eccentric scientist helps out a high school boy.

While he would never advise any other man to dress like Sanders, Mackie said, it is the perfect look for Bernie. “His shirt is billowy, his hair is unkempt, and you look at his jacket — very heavily shoulder-padded, and they’re falling off his shoulders. I see a correlation between that and the Internet culture — all these start-ups where you can literally wear anything you want to the office, but there’s the perception that they work even harder because they’re not focused on those trivial things. I would never tell Bernie, ‘Bernie, you ought to clean it up.’ He’s got that disheveled thing going, but he speaks the truth, and that’s cool. . . . I think if he were buttoned up it would hurt him, it would eat into his message.”

Getting behind an older candidate may also be less of a stretch for a generation of young people who tend to be tighter with their parents than previous generations were. “They enjoy interacting with their grandparents and sharing new things, and showing them how to use the computer and having intellectual conversations with people with whom they have a large age gap,” Collinson said. They also put more time into helping them: Nearly one-quarter of America’s adult caregivers are between 18 and 34.

Perhaps nowhere is millennials’ embrace of the old more evident than in their love of all things retro, from vinyl records to mountain-man beards. A No. 1 rap song a couple of years ago — “Thrift Shop” — even extolled geezer chic: “I wear your granddad’s clothes/ I look incredible.”

“We’ve seen so much shoddy crap come into our country,” Mackie said. “I do think that there is a harkening back to a time when things were made better.”

A sort of Make America Great Again, Bernie-style.

Sometimes, to quote the millennial bard Justin Timberlake, what goes around comes around. Sanders and his young supporters are so far apart in age that, in a sense, they converge. Millennials don’t associate the word “socialist” with the Iron Curtain; to them it evokes Scandinavia, with its short workdays and generous parental leave. Sanders, for his part, dates to a more “socialist” period in the United States, when welfare programs were implemented and veterans went to college on the G.I. Bill.

To Bethany Vance, 20, a sophomore at the University of North Carolina at Chapel Hill, Sanders is more of a kindred spirit than baby boomers are. “Someone who’s 60 looks at me and says, ‘Why are you complaining?’ ” she said. “Prices have gone up. They don’t see that, but he does. . . . A lot of people feel that someone so old would be out of date, but everything he’s saying is timeless.”

But what about the stamina required to be president? The job is famous for turning people’s hair gray — what would it do to someone whose hair is already white, someone who would be 83 at the end of a second term?

His supporters shrug.

“Senator Sanders’s ability to work through a national campaign schedule and get back to the Senate for votes has been a promising display of his resilience and tenacity despite his age,” Noonan said.

Perhaps in an era when the Rolling Stones have been touring for more than 50 years, the idea of an energetic septuagenarian in the White House is not so outlandish.

“Even though he’s pretty old, he has a lot of youth to him,” said Britta Galanis, a 20-year-old Sanders supporter.

Collinson agreed, adding, “That whole discourse” — of Sanders being too old for the job — “feels a little bit ageist to me.”