It’s been nearly two decades since the psychologist Jeffrey Jensen Arnett coined the phrase “emerging adulthood” to describe young people between the ages of 18 and 29. Previous generations, Arnett observed, moved from adolescence into adulthood without much preamble; it used to be common to marry right out of high school, start a family, and work a steady job.

But then things started to shift. Arnett observed young adults milling about the waiting room before entering adulthood, compounded by an uncertainty about how exactly to start “adulting.” And yes, that verb is now in regular, if still somewhat self-conscious, circulation, thanks to a website of the same name that promises to teach young people “how to become a grown-up,” “in 468 easy(ish) steps.”

Around the time Arnett published his ground-breaking work, Britney Spears, the poet of emerging adulthood, wrote a song called “Not a Girl,” a song which could be considered the ballad of the in-betweeners.

I’m not a girl / Not yet a woman
(I’m not a girl don’t tell me what to believe)
All I need is time / A moment that is mine
While I'm in between

Neither Arnett nor Spears probably fully understood the development that would end up reshaping emerging adulthood more than any other: the emergence of the Internet as the dominant social context of our time. The Internet today is a pervasive presence, a thoroughgoing part of existence. Young adults still experience the intensity of being in-between, living in the “age of identity exploration,” as Arnett put it—only now they experience it online.

It is easy to forget just how new, and how startling, this shift is. In the past, ...

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