Demonstrators hold up placards with slogans against the split with the EU at an anti-Brexit protest in Trafalgar Square in central London on June 28, 2016. (Justin Tallis / AFP Getty Images)

It's like the last twist in a horror movie. You think the baby boomers are dead, or at least easing into retirement, and then they thrust their claw up from the dirt, or lunge from the bathtub with a knife. Or take over a presidential election. Or decide to "liberate" your country from an economic partnership that is imperfect but deeply popular among younger generations whose vision is more kaleidoscopic than the color scheme of a national flag.

"YOU STOLE OUR FUTURE FROM US," said a blue banner hoisted by two young women in front of Big Ben after the United Kingdom voted to "Brexit" the European Union last week.

The "YOU," of course, are the olds. The pensioners and soon-to-be pensioners. Voter turnout among Brits over 55 years old was 82 percent, according to the BBC. Among Brits under 34, it was 47 percent. If at least 65 percent of British millennials had turned out, the outcome would probably have been different.

A more accurate banner then: "WE LET YOU STEAL OUR FUTURE."

Whose future is it anyway? The politician who's approaching his or her life expectancy? Or the first-time voter who might have another 70 years on the planet?

Forget class warfare. There was generational warfare on Twitter after the Brexit vote.

Millennials vs. baby boomers! The war has been ongoing stateside since at least 2008, when the olds inflicted the Great Recession on the youths. Now, in the form of either Hillary Clinton or Donald Trump, the boomers want another eight years in the White House they've held for a quarter century. Millennials were so galled that they ran into the arms of someone even older, someone who was born before the attack on Pearl Harbor but who shouted himself hoarse, which is ironic because Bernie Sanders is technically a member of the Silent Generation. The three remaining candidates for president are the oldest group of presidential candidates ever. Think about that!

Donald Trump and Hillary Clinton were born at the very beginning of the baby boom, in those flush postwar years; they came of age in the '60s and '70s, as America lost its mind on the Perfume River, on the balcony of the Lorraine Motel, on the campus of Kent State. "Don't trust anyone over 30" was a boomer mantra in the 1960s. They put it on buttons as they railed against the Man.

Now they are the Man. Trump turned 70 this month. Hillary Clinton turns 70 in what could be her first year in office. Republicans have long been the party of the olds — the Republican nominee has been younger than his Democratic opponent in only two elections since 1952 — but Clinton will be the first Democratic nominee for president over the age of 60 since Harry S. Truman. Trump will be the third septuagenarian Republican nominee for president in the last six elections; he'd be the first-ever septuagenarian president at inauguration.

There may be multiple choices this November, but everyone will be voting old. Even third-party candidates Jill Stein and Gary Johnson are boomers (and Johnson's running mate William Weld is a year older than Trump).

[You will barely recognize Hillary Clinton and Donald Trump in these old interviews]

With the exception of the 2000 and 2004 elections, Americans are used to picking between an older guy and a younger guy for president. The younger guy, if he prevails, is credited with pulling America into a new era. Barack Obama was born in 1961, at the tail end of the Baby Boom, on the cusp of Generation X, but his relative youth, background, and inexperience on the national stage gave him an aura of newness, of next-ness, of we're-moving-beyond-all-this. John F. Kennedy, the first U.S. president born in the 20th century, was nearly 27 years younger than his predecessor Dwight D. Eisenhower; his mere arrival re-mythologized Washington as Camelot. At the time Bill Clinton picked Al Gore to be his running mate, they were 45 and 44 years old. Articles were written about how boomers were finally wresting power from the Greatest Generation, embodied by the vanquished George H.W. Bush, who was a pilot in World War II before Clinton was even born.

"To other generations, Baby Boomers have become a plague on the rest of the country," wrote The Post's Dan Balz in April 1992, as Bill was fighting Jerry Brown for the Democratic nomination.

The angst! The self-absorption! The paranoia and greed and psychic scar tissue from Vietnam! Bill's campaign song was Fleetwood Mac's "Don't Stop," a forward-looking anthem about tomorrow that was already 15 years old at the time.

"Yesterday's gone," Lindsey Buckingham sang. "Yesterday's gone."

Except it's not, and never is. Fleetwood Mac is still touring; last year they were the sixth-highest-grossing concert act, a few spots behind the Rolling Stones and a few spots ahead of the Grateful Dead. And former Goldwater girl Hillary Clinton, 68, is leading the polls. It's 2016, mind you. This was supposed to be the year of the Gen-X candidate. Ted Cruz, Marco Rubio and Paul Ryan were born between January 1970 and May 1971, grew up in Reagan's shining city on a hill, inherited the Pax Americana of the '90s. But Boomers and Millennials have a way of sucking the oxygen out of the room, which gives Gen X another reason to feel disaffected.

When Clinton declared her candidacy last year, millennials had just become the greatest generation, population-wise. They (we) have been as documented and derided as the baby boomers. The angst! The self-absorption! The entitlement and heterodoxy and psychic scar tissue from watching the towers come down! Our heads are in the Cloud. Our brain is in our phones. It's never been easier to create our own virtual world at the expense of the one we actually live in.

"We're happy, free, confused, and lonely at the same time," wrote the poet Taylor Swift, 26. "It's miserable and magical."

Our elders have broken the tradition of being good ancestors. According to them, they have left us worse off: unable to afford college or a house, hitched to rising national debt and global temperatures, forced to create movements to convince them that our lives matter. And yet only half of Americans under 30 say they are certain to vote this fall, according to a poll conducted this month by The Post, while the level of certainty is nearly 80 percent for American voters over 65. The older someone is, up until age 75, the more likely he or she is to vote. Why is that? Shouldn't a desire to shape the world decrease alongside one's time left in it? Or does mortality induce a sly panic, a zombie patriotism, an increasing desire to leave one more mark, however small?

A lot of small marks can add up to a big one. One reason for the rise in the millennial population is immigration, which is precisely what triggered the Brexit referendum: a fear that what is will soon no longer be. Which sounds a lot like the fear of death. Strange, then, that the medium of the moment is Snapchat, which is all about making things that don't last. The Clinton campaign is very good at using Snapchat. The campaign sends video and images that disappear in the hands of young people who probably won't vote. It's 2016, definitely.

The soonest a millennial will be a major-party nominee is in 2028, if we're going by precedent. The youngest boomers will then be cruising through their mid-60s. So perhaps this generational warfare will actually reach the ballot box that year, in the form of millennial candidate vs. boomer candidate. By then, at least, millennials should be old enough to want to vote.