In less than 100 days, the people of Scotland will determine whether they should break off from the United Kingdom and become independent. The referendum on Sept. 18 is a once-in-a-lifetime choice for young Scots. (Nicki DeMarco and Griff Witte/The Washington Post)

— Before heading to college this fall, 17-year-old Gavin Lundy has plans both ordinary and revolutionary. The young Scotsman wants to make some money working at a cafe. And in his free time, he intends to help forge the world’s newest independent nation.

“The people of Scotland have never had this power,” said Lundy, a wave of blond hair framing a gaze fixed with intensity. “But now we have the chance to build a radically new and better country.”

Or not.

To be young and Scottish this summer is to be swept up in the thrall of a once-in-a-lifetime choice: a September referendum that will determine whether Scotland sticks it out with the United Kingdom or jettisons a three-century-old partnership and goes it alone. The outcome holds vast repercussions for Britain and for the United States, which wants to keep its closest and strongest ally united.

With less than 100 days to go, opinion polls show that most Scots agree, even among the 16- and 17-year-olds who will be allowed to cast a ballot for the first time. But in the gothic streets of Edinburgh, it’s the pro-independence forces­ that seem to have the upper hand in energy and enthusiasm, a disparity that’s most apparent among the young.

Generation Yes campaigners pass out leaflets for the Scottish independence referendum in Glasgow. A referendum on whether Scotland should be an independent country will take place Sept. 18. (Jeff J. Mitchell/Getty Images)

As with independence struggles the world over, this one has attracted armies of true-believers burning with desire to make their homeland anew. Unlike most, Scotland’s separation from the United Kingdom could be achieved without a shot fired as teens and 20-somethings wield IMF reports, comedy skits and Twitter memes to evangelize their case for doing what generations of brawny Scottish warriors couldn’t — breaking from London once and for all.

“The independence campaign used to be seen as ‘Braveheart’ and bagpipes,” said Graeme Sneddon, 22, a part-time coordinator of the Yes Scotland campaign’s youth wing and a full-time grad student who will never be confused with Mel Gibson’s depiction of 13th-century Scottish freedom fighter William Wallace. “We’ve gotten to the point where people can look beyond that and talk about the issues.”

Sneddon, like many independence activists, had no background in politics before the campaign. But he investigated the arguments for and against secession with the same rigor he brings to his long hours in the lab studying plant biology and came to an inescapable conclusion: Scotland would be better off on its own.

“The other side says, ‘You’re too small. You’re too daft. You’ll make mistakes,’ ” Sneddon said. “But certainly the people of Scotland can’t be any worse at running our affairs than Westminster.”

Westminster, home to the British Parliament, is a dirty word among independence advocates — a stand-in for the disconnect between a Conservative-led government that pulls increasingly to the right and the will of Scottish voters, who vote consistently for parties of the left.

Choosing independence, Yes campaigners say, would mean never having to live under a Tory government again while enabling the 5 million people of Scotland to build a more just and egalitarian society.

“Independence gives us the chance to radically transform child care and to start tackling poverty,” said Sneddon, who cites the Scandinavian countries as models of small nations that have provided their citizens with generous social-welfare programs. “We have a wealthy economy, but the people of Scotland aren’t feeling the benefits of that wealth.”

He’s planning to spend his summer making that case across this relentlessly green land, preaching the virtues of independence on doorsteps and online.

Few doubt that the Yes campaign has the more-committed activists, their utopian visions of a reborn Scotland ringing louder than the sober calls for steady as she goes. Independence advocates are likely to dominate the debate at Edinburgh’s annual Fringe Festival in August and are already lighting up the Web with viral videos that mock the often-stilted attempts by the unionist campaign to appeal to young voters.

Yet so far, most Scots remain unconvinced about taking the leap. The independence campaign has gained ground this year, narrowing the unionist lead to single digits in some polls. But there remains a substantial gap to make up between now and Sept. 18 if the dream of independence is to become a reality.

Surprisingly to many political analysts, surveys show that 16- and 17-year-olds have been among the most hesitant to ditch the United Kingdom. The Scottish Parliament, which already has considerable authority over local affairs and is led by Scottish nationalists, opted last year to lower the voting age for the referendum, reasoning that teens will have to live with the consequences longer than others.

All signs suggest those under 18 are embracing their new powers with gusto — a staggering 80 percent of the 120,000 newly eligible voters are already registered, and surveys show they may be following the debate closer than adults.

“Young people are very active and engaged because it’s going to affect them more than any other generation,” said Kyle Thornton, 19, chair of the Scottish Youth Parliament, a nonpartisan group that advocates for young people. “They’re thinking about the long-term effects of the vote. What will the jobs of the future look like?”

That’s what worries John Morgan, a 22-year-old lawyer who’s active in the unionist cause. Putting up a barrier between Scotland and the rest of the United Kingdom, Morgan said, will only hurt young people’s employment opportunities and limit their horizons.

“It’s the antithesis of what we’ve been taught about globalization and how we’ve learned to live our lives,” he said.

Morgan’s side recently received a boost when President Obama broke with long-standing U.S. policy and weighed into the debate, saying the United States has “a deep ­interest in making sure that one of the closest allies we will ever have remains a strong, robust, united and effective partner.”

The unionist camp received another key endorsement last month when author J.K. Rowling — an Edinburgh resident and author of the Harry Potter books on which millions of millennials were reared — dropped $1.7 million into the Better Together campaign’s coffers and compared some Yes advocates to her fictional “Death Eaters” for focusing on “the purity of your lineage.”

But for the most ardent young believers in Scottish independence, the debate has little to do with ethnic nationalism. Young Yes activists mock British Prime Minister David Cameron for trying to seduce voters by invoking his Scottish roots and his affection for Scotch whisky.

Indeed, unlike many independence debates, this one isn’t dictated by ethnicity, race, language or religion. Instead, it’s up to each Scot to choose after a debate that, so far at least, has been remarkable for its civility given the considerable stakes.

“I’ve yet to see a smashed window over this, let alone a pub brawl or a riot,” said Michael Rosie, a sociologist at the University of Edinburgh. “Young people are weighing the facts and thinking very carefully. It doesn’t fit the stereotype of young people not caring or not thinking about the consequences of their actions.”

Peter Geoghegan contributed to this report.