It seems like only recently that people in their twenties became the focus of sustained cultural fascination, or self-fascination, but perhaps that’s always been the case. “Few decades of experience command such dazzled interest,” Nathan Heller wrote in The New Yorker, a couple years ago, in a roundup of the latest haul of twentysomething-themed books. On an episode of “Girls” not long ago, someone told Hannah Horvath, in an effort to cheer her up, “You’ve lived all this truth.” She replied, “It didn’t feel like very much was happening.” So it goes for so many twentysomethings, at least the ones fortunate enough to stumble around looking for a plan: life experiences—big, small, weird, disappointing, joyous—accumulate, like puzzle pieces that don’t quite fit together. And then, one day, the twenties are over, and one has, for better or worse, an adult life.

For the past two and a half years, I’ve immersed myself in the twenties of a distinctly uncommon person: Meryl Streep, the subject of my book, “Her Again: Becoming Meryl Streep.” It traces Streep’s life from her suburban New Jersey high school (she was the homecoming queen, naturally) to her breakout roles in “The Deer Hunter,” “Manhattan,” and “Kramer vs. Kramer,” for which she won her first Academy Award, at age thirty. I’d long been obsessed with the swanning diva we know now, the “greatest living actress” who calls out Hollywood sexism and gives flawless acceptance speeches that manage to be self-deprecating and grand at the exact same time. The question I asked myself at the outset was this: Who was Meryl Streep before she was the unsinkable queen of acting? Was she ever just an aimless twentysomething, trying to make her way in the world?

Yes and no. Meryl Streep was always, on some level, Meryl Streep: a woman of uncanny ability and drive, blessed with a superhuman sense of self-possession. But she didn’t always know how to apply her gifts. In high school, she modelled herself on the women she saw in fashion magazines, and acted the part of a conformist all-American popular girl: joining the cheerleading squad, dyeing her hair golden blond, dating football players. It was only when she got to Vassar, in 1967, that her world opened up and she discovered feminism, acting, and something like an authentic self. She was twenty when she played her first serious role, the title character in Strindberg’s “Miss Julie,” in a 1969 college production. “It was a very serious play, and I had no idea what I was doing, really none,” she said later. “But oh, my God, it was just a place to tap into all sorts of feelings that I had never had, I guess, admitted to myself, or felt like parading in front of a group of people.”

By the end of the seventies, that awestruck novice had become the star student at the Yale School of Drama, had headlined on Broadway and in Shakespeare in the Park, and had filmed her Oscar-winning role in “Kramer vs. Kramer.” She had found and lost her first great love—John Cazale, the character actor who played Fredo in “The Godfather,” and who died, of lung cancer, with Streep at his bedside—and then, a mere six months later, married the second great love of her life, the sculptor Don Gummer. When she turned thirty, in 1979, she was pregnant with their first child, Henry. It goes without saying that few people’s twenties tie up in such a tidy bow: a marriage, a baby, and an Academy Award. The challenge, for me, was to make what seems inevitable in hindsight feel wildly improbable in the moment.

And that’s where I gained some perspective on my own twenties, which came to their scheduled conclusion almost five years ago. As I was writing, I was aware of the strange fact that my main character was both older and younger than I was. Streep is sixty-six now, having lived a life of international celebrity for several decades, and yet the person I was studying was an unknown, untested twentysomething still defining herself in ways that felt familiar, because I’d already lived them. There was the dreamy college freshman who had overpowering reactions to books and art and music. In letters to her former high-school sweetheart, a medic in the Vietnam War, she wrote from her dorm room about seeing Simon and Garfunkel at Dartmouth; when Garfunkel sang the last line of “For Emily, Whenever I May Find Her,” it was “almost like the beautiful feeling you have when someone first tells you the same thing.” The day she finished reading “A Portrait of the Artist as a Young Man,” she thought she was having a “severe identity crisis.” I knew what it was like to be twenty and get dizzily lost in a song (during college, I once listened to “Mona Lisas and Mad Hatters” on a loop for three hours), or in James Joyce, or in a momentary “identity crisis.”

There was Meryl at twenty-three, still wondering if she should become an environmental lawyer while she was enrolled at the Yale School of Drama, until she slept through the exam. (I applied to film school at twenty-two, got in, and then shocked myself by not going.) There was Meryl at twenty-six, who had just moved to New York and had an apartment and a roommate and told herself, “I’m starting my career. I better make it next year.” (I knew the feeling of wanting it all to happen right away, even as you’re still learning the rules.) Then there was Meryl at twenty-eight, throwing everything aside to care for Cazale, and then losing him—her first devastating tangle with grief. I knew glimmers of that story, too. At twenty-five, a close friend of mine, who was likely suffering from undiagnosed schizophrenia, hurled himself out of a window. Scattering a vial of his ashes in Coney Island, I told a friend, “This has aged us.” I remember feeling passive, like the most consequential things in life were the ones that happened to me, sometimes for no apparent reason. When I was twenty-four, I was crossing the street near the South Street Seaport and the side-view mirror of a moving bus whacked me across the face. (I was listening to a jaunty Jacques Brel song on my iPod, which, bizarrely, kept playing after I was flung onto a parked car.) A surgeon had to put in a titanium plate in my left cheek. That didn’t fit into any story I had concocted about my life, but it seemed to matter, somehow.

All this is not to say that my life is somehow eerily parallel to Meryl Streep’s. I have yet to be named the best actress of my generation, despite several groundbreaking karaoke performances. When I look back at my twenties now, I can see that they were all leading somewhere. As a story, they make sense. But at the time, as Hannah Horvath says, it didn’t feel like very much was happening. Most things felt haphazard, random, peripheral. I was making decisions, sometimes, without realizing it. Other times, they hit me square in the face, like that bus. Warning: this is the part where I quote Joan Didion. “That was the year, my twenty-eighth,” she wrote in “Goodbye to All That,” the urtext of twenties self-reflection, “when I was discovering that not all of the promises would be kept, that some things are in fact irrevocable and that it had counted after all, every evasion and every procrastination, every mistake, every word, all of it.” The twenties “count,” though they don’t always feel that way. You sleep through a law exam, cross the street listening to Jacques Brel, and it counts. It’s only when they’re over that you can see the shape of things.