For decades, parents understood that you had a child for only 18 years before releasing the dear one into the world.

Now, if you’re lucky, you can unload the little darling by age 27.

This condition has several clinical terms – “prolonged adolescence” or “delayed adulthood” – though the most appropriate might be “free rent plus laundry privileges.”

A friend brags about how her 26-year-old daughter and beau finally moved out of the family home and rented their own apartment, crowing as if the child had landed a residency in neurology. A mother announces her progeny has finally “cut her off” financially, won’t accept another cent, now that she’s finished college on the nine-year plan.

A decade or two ago, twentysomethings were mortified to share the same roof as their parents, as sure a sign of loserdom as bad hair. Sure, we loafed. Twentysomethings are supremely gifted at loafing. They’re hardwired that way. But we loafed backpacking across the country or living in group houses.

Initially, it seemed odd that anyone might extend adolescence; a fractious, frequently unhappy phase marked by confounding hormonal swings and social unease. The best way to get through adolescence then, as now, was via copious amounts of junk food, loud music and questionable aesthetic statements.

On closer examination, though, the reasons for holding maturity in abeyance are obvious. College costs are punitive, yet necessary to a better life, as is health care. Bad jobs with no benefits are plentiful while good jobs, without the proper education, are difficult to secure. People are marrying later. Women are having children later.

And what’s the rush? Most adults don’t look as if they’re having much fun.

That’s because most adults aren’t.

The University of Pennsylvania’s Network on Transitions to Adulthood, led by sociologist Frank F. Furstenberg Jr., has done terrific work on the matter. The team identifies adulthood by five benchmarks: completing school, getting married, having a child, becoming financially independent, and leaving home.

Some passages no longer suggest adulthood. Any child can have a child, thereby obviating the other accomplishments. For some, it takes little maturity to marry at an early age, only a good divorce lawyer, as Britney Spears or Jessica Simpson can attest. Meanwhile, completing school often ends in a puddle of debt.

Small wonder children are in no hurry to acquire utility bills, given the rapture they induce, some outstripping car payments.

Adulthood requires new benchmarks to make it more alluring and ease the transition. An adult financially assists others, makes charitable contributions, picks up the check. An adult purchases her own linens, cooks for other people, gets up in the morning without a parent’s prodding.

An adult returns phone calls and sends thank-you notes. An adult makes plans and keeps them.

An adult knows what she’s drinking. She knows the full name of her dates.

She trends toward responsibility, and, unlike many adolescents, thinks about other people’s happiness.

An adult avoids debt, which has an annoying way of becoming a problem for other people.

An adult does the dishes, knows how to operate a dryer.

An adult shows up on time.

And never breaks up with someone via e-mail.

The problem with living longer is that everything lasts longer, a conundrum in a culture where so much is done with alacrity and little care.

If adolescence is prolonged to span a decade and a half, even two, imagine what’s happening to middle age, a fractious, frequently unhappy phase marked by confounding hormonal swings and social unease.

Already, signs suggest middle age isn’t going to be the middle anymore. It’s going to last forever. It’s going to be the end of many of us, which no amount of junk food, loud music, and questionable aesthetic statements can alter, making a prolonged adolescence look more fetching by the hour.