For his first six years in New York City, Ben Grow lived out a particular urban fantasy. He and a group of other classical musicians, several of them friends from college, all found apartments in the same Washington Heights building; at one point, there were nine of them spread across three units. Living together, the friends became a community, sharing their craft and recommending each other for gigs. Sometimes, their Friday-night jam sessions devolved into the sort of aural debauchery you’d expect of classical musicians after a few beers. They’d take a Vivaldi aria, for instance, and “do it in the style of Mariah Carey,” Grow recalls. The camaraderie, not to mention the rent—Grow paid a mere 600 bucks a month—more than made up for a shifty superintendent and the colony of feral cats in the basement.

And then, inevitably, the group split up. Grow was nearing thirty. He’d started making more money conducting and was itching for more room for his instruments, musical scores and reference books. Meanwhile, one of the guys in his apartment announced he was moving to Florida. The other was getting serious with his girlfriend. Their run was coming to an end.

Grow moved to Inwood, and for twice the rent, got a junior one bedroom with all the trappings of true adulthood: plentiful refrigerator space, a cleaner bathroom, and tons of…quiet. It was what he wanted, but it was bittersweet. No more spontaneous roommate bike rides, no more skipping trumpet practice to see dumb movies together. Now, to see those same friends, he had to “plan and coordinate and build in travel time.”

Young people have always had roommates, of course. What’s new is how long today’s young people are living in such arrangements. Once a temporary holdover from college life, platonic cohabitation is now carrying some city dwellers through their twenties and into their thirties. Being a roommate becomes a significant (if subconscious) part of their identities; for those lucky enough to find a stable, long-term roommate setup, it also provides a defining partnership. All of which makes the eventual break-up of these sustained roommate relationships a new rite of passage.

In the best circumstances, roommates finish growing up together. They see the aftermath of bad dates and are the first to meet future spouses. They nurture each other’s career ambitions and witness the attendant concessions. They are friends and economic partners and fashion advisors and a guarantee against lonely weekends. As Catherine Grenfell, a 28-year-old Mississippi native, says of her four long-time New York City roommates: “We became each other’s family.”

In June, Grenfell’s Tribeca fivesome broke up. On move-out day, amid the accumulated stuff of six years, she found herself bawling. “We made a lot of memories in that apartment.”

Census data shows that roommates are a small but growing subset of the American population, part of a larger trend toward unrelated adults living together. The increase may be most pronounced in expensive urban markets cities; the New York Times has reported that between 2000 and 2010, the “number of roommates in nonfamily households” in New York City jumped by over 40 percent. While data measuring the length of roommate arrangements is hard to come by, social demographer Frances Goldscheider of the University of Maryland says it stands to reason that people are living as roommates longer, given a shaky economy, sky-high rents in cities like New York, Seattle, San Francisco and Los Angeles, and the fact that Americans are marrying at progressively older ages.

Those on the other side of roommate breakups talk about their wistfulness for the old days as well as the stunting effect of living with the same person or people since they were eligible to drink. If college graduation once served as the bright line between youth and adulthood, now shared apartments can serve as the setting for the extended adolescences that are the new norm. There’s the filched Gatorade, the missed rent checks, the living room sex, the overnight guests who never leave. Most of all, there’s the mess, which for the people doing the leaving tends to be symbolic of who’s matured and who has not.

Fairchild Fries, 31, is a soon-to-be ex-roommate. He hasn’t yet told his close friend Matt that he’s planning to move out of their apartment on New York’s Lower East Side, though he thinks Matt probably suspects something. The two have been living together, along with a succession of other roommates, since sophomore year in college 12 years ago. They went through their young and crazy phase together; at one point they were part of a group of nine people sharing a three-bedroom apartment, where it was not unheard of for someone to fall asleep in the bathtub.

But now, says Fries, an ad agency design director, their roommate-dom has run its course. Matt is still in party mode, and Fries has begun to “feel like a mom,” telling Matt and their other roommate that “two minutes in the bathroom” doesn’t constitute a good cleaning. (“We’re bros,” Fries says. “Things happen in the bathroom.”) He wants a quieter, cleaner, more adult existence, one in which he listens to NPR in the morning instead of hearing the constant drone of SportsCenter. But first, he has to get through the logistics of leaving: finding a new place, telling Matt, splitting all their stuff.

It will “be like a divorce,” Fries says. “We own a lot of things together…What happens when you buy a $1,000 TV and you’ve got to split it?” He goes on: “It sucks, ‘cause you’re talking about money and you’re talking about property, and I don’t know what Matt’s reaction is going to be to that.”

The marriage analogy only goes so far. Spouses are supposed to take for granted that their careers may not progress at the same rate, but for roommates, one person’s promotion usually spells the end of the partnership. Suddenly one can afford the upgrade to a much nice neighborhood while the other winds up weeding through creeps on Craigslist. Sometimes, group houses are depopulated by a series of friends hitching up, leaving behind people who feel doubly rejected—first by their roommates and then again by their last boy- or girlfriend. Sometimes, amid the turbulence of an early career, a great roommate of five years decides to take a job in, say, Bolivia.

“That was heartbreaking,” says Dave Ratzlow, 44, who lived in an East Village apartment with a succession of good and bad roommates for fifteen years. He’s remembering one of the good ones. “We played pool every night, we would try to pick up women, we would go to shows, we would make music together. Our lives were intertwined.”

And sometimes the precipitating events are just an excuse for what everyone knows has to happen. “We all knew in our heart of hearts” that it was time break up, Grenfell says, though it took a former roommate—who’d left the apartment to get married a few years before—to state the obvious. “She said it at dinner one night: ‘You guys all need to separate and make a change and experience a different New York,’” says Grenfell, who works in sales for a hotel company. “It gets to a point where it’s maybe a little bit unhealthy.”

Two women in the group moved in with their boyfriends, while Grenfell and another roommate made their plans to relocate without telling their fifth roommate, who had no job and from whom they’d grown apart. (“In nice, polite, southern fashion,” Grenfell says, “we talked behind each other’s backs, on Gchat.”)

But both the leaving and the left-behind generally seem to sense that there is no going back after a roommate break-up, precisely because it is as much about a time in life as it is about saving money on the rent. Grow, the conductor, talks of his old place in Washington Heights less as a place or a group of people, and more as an era that’s over.

“I’m not sure it would be the same if I were there again,” he says. Something’s changed, and it’s about more than geography or privacy or space for his piano. It’s him.

For some, though, room-mating seems to get into the blood. As Fries works up the nerve to split with Matt, he has help searching for his next place. He’s planning to move in with his brother.