“When something itches, my dear sir,” a psychiatrist advises the middle-age, married protagonist struggling with his attraction to a beautiful neighbor in the classic film “The Seven Year Itch,” “the natural tendency is to scratch.”

Five decades ago, the phenomenon known as the seven-year itch was considered a turning point when husbands and wives battled the temptations of extramarital lust or love. Today’s unmarried twentysomething couples are just as young and just as restless, and, though they’re unmarried, they often live together. But they have a different kind of itch: Their longing for the single life is as common as the seven-year itch is for those knee-deep in marriage.

Reporter David Odegard’s three-year relationship ended soon after the lease expired on the apartment he shared with his college girlfriend, when he was 25 and she 26. Lawyer Kalpana Nagampalli spent years negotiating long distance and larger differences with her boyfriend before ending it when she was 24. Public-relations executive Julie Ma and her ex — the “faraway crush” she had first eyed in their freshman-year dining hall — dated for six years yet called it quits months ago, at 25.

In 1955, when “The Seven Year Itch” was released, the average married age was about 22 for men and 20 for women, according to the Census. But today, the mid-20s is often a period of extended adolescence: An enticing mixed bag of incentives — from a provision in the new health-care bill allowing parents to insure children until age 26, to the variety afforded by online dating — tempts many to split with longtime significant others, and, in effect, to help keep the median age of marriage increasing. Last year’s average, incidentally, was 28 for men and about 26 for women.

Psychoanalyst Dr. Lawrence Birnbach and his wife, consultant Dr. Beverly Hyman, studied the trend while researching their guide to breakups and divorce, “How to Know If It’s Time to Go.” In a telephone interview, the pair suggests that splits prompted by age 25 can be attributed to a collection of mindsets and personalities that could be divided into groups: the space-seekers, the commitment-phobics, the super-ambitious and the impractical.

For example, those seeking space, says Hyman, “announce to a long-term girlfriend or boyfriend, ‘I’m going to take six months,’ or ‘I’m going to grad school on the other end of the country,’ or ‘I want to take a job in another country.’”

Diana Bejasa, a 25-year-old chef, attended culinary school in New York near her boyfriend, whom she had begun dating at their Manhattan high school. “We were growing apart and wanted different things in life. I was about to graduate and thinking about what I wanted to do next. I knew I wanted to travel and live all over the world,” she says. “And I knew I couldn’t do that if we were together.” Their seven-year relationship ended, and she moved to San Francisco soon after graduation.

The authors describe the second mindset as “commitment-phobic. They feel less inclined to tolerate something they don’t like than try to work it out,” says Birnbach.

Odegard, 27, a media reporter, moved in with his college girlfriend after graduation. But when he began graduate school, they spent little time together and slowly transformed, he says, into roommates. After their lease ended and he moved out, he says they still harbored fantasies of making it work, until the day he realized it had ended: “I had just loaded all my belongings and furniture into a U-Haul and moved it to Brooklyn, and I came back to where we had been living together for years and it was a half-empty mess. The cats were freaking out because the furniture was gone. I was sitting on the floor because there was nowhere else.” He says, “It hit me. I knew it was over.” Odegard has been dating his current girlfriend for over a year.

There are also the super-ambitious, those who “put legendary amounts of hours into their work. But it’s very tough to nurture a relationship if you’re away all the time,” says Hyman.

Nagampalli, a lawyer from India, had dated her college boyfriend since age 19. But things turned sour when she moved to Philadelphia to attend law school. “He had a very limited view of the world and expected me to kowtow to a lot of things,” she says. “He would call me all the time when I was at a class, wanted control of my e-mail,” she says. By moving, she “had ruined his life. I couldn’t take the pressure. More importantly, I had stepped out of the circle of the relationship and noticed I was having more fun without him.” Nagampalli has been with her current boyfriend, whom she says has only one thing in common with her ex — they’re both lawyers — for three years now.

And lastly, the experts note, there are the impractical. Those who “have an unrealistic view of what a relationship is like,” say Birnbach. “Even if you’re young and free and beautiful and having a great time, things get a little old and you do fall into ruts,” chimes in Hyman. “People at every age become disillusioned and think, ‘There’s gotta be more than this!’ But they don’t know how to take the next step. It’s easier to walk away.”

Julie Ma and her boyfriend managed distance — she’s from Oregon, he’s from Alaska — and holidays apart, but also jointly owned a dog and spent a summer living together. She says via e-mail, “We had the type of relationship that most of our close friends absolutely envied. They said, ‘If you and him don’t make it, then who’s going to?’” But ultimately they didn’t. “Whatever opportunities were presented to us in life, we were going to take them one by one,” she says. “With one exceptional rule: Don’t let the other person hold you back.” It may have been this decree that did them in, she reflects, “To this day, I feel like, maybe that rule broke us a bit.”

Of course, young couples do make it work despite the disillusionment that distance brings or the appeal that new prospects present. Danielle Patacky, 26, and her boyfriend Mark met through a mutual friend at Arizona State University. Nearly seven years, one cross-country move (from Arizona to Connecticut), and two high-intensity jobs (hers in event planning; his in investment banking) later, the pair is still intact.

Patacky explains their secret: “We don’t spend every second together. It makes the time we do spend together very special.” Still, she admits, “These are my 20s, ‘the best years of my life,’ according to my mother. And not knowing for sure that the person you’ve spent these years with is going to propose can be unsettling. How do I overcome this?” She answers herself: “Patience, patience, patience. And knowing that he is the one.”

For some, though, there are no easy explanations lurking within the rubble of a recently broken heart. Kiahni Vann, an Atlanta-based psychotherapist who not only primarily treats twentysomethings but, at 27, happens to be one, says by phone, “Meeting someone when you’re going through the motions of life is one thing, but as you grow up, you start living your life by your own rules.” Sometimes, she adds simply, “He, or she, may just not fit into that.”