Sandy, the heroine of “Wifey,” tried to escape her circumstances through fantasy. Blume worked her way out of her marriage by asserting herself through writing. She took a class in how to write for children, which led to her first novel: “Iggie’s House,” about a young white girl befriending black children who had recently moved to her nearly all-white neighborhood. Before that book was published in 1970, she had already started on what would turn out to be “Are You There God?”

Blume is verbal and warm and open, but she says that her son has called her “the least analytical person he has ever met.” She has no theories, for example, to explain why she, of all people, felt unburdened by the unspoken rules marking certain subjects off limits for children, or why, for that matter, she has that particular gift, that ability to recall the emotional experiences of adolescence, the confusion, the longing, the rivalries — the memories, in other words, that most of us try to bury as quickly and deeply as we can.

Blume does think that she turned toward children’s fiction because she was still living a relatively sheltered life. “I didn’t have any adult experience when I started to write,” she said. “So I identified more with kids.” Her own fate felt sealed, airless. “I felt, I made this decision. This is it. It’s not all open for me anymore.” To her, it was only natural that she look backward, to the age when she felt most powerful and adulthood still promised the adventures her father wanted for her. She had been a fierce and creative child; on the page, at least, she still was. Blume likes the idea that everybody has an age that defines them for life. For her, she said, that age is 12.

In Key West, Blume is friendly with Meg Cabot, the young-adult author best known for the “Princess Diaries” series and a writer who grew up reading Blume’s work. Their friendship has moved beyond a mentor-student role, mostly because Blume prefers it that way. ”I’ll be like, Judy, however did you write that masterpiece, ‘Blubber’?” Cabot said. “And she’ll be like, ‘Who do you get to trim your palm fronds, because mine are driving me crazy.’ With us, it’s all island gossip and landscaping.”

Cabot was one of 24 women who contributed to a 2007 anthology of tributes to Blume’s work called, “Everything I Need to Know About Being a Girl I Learned From Judy Blume.” Young-adult novelists, in particular, celebrate Blume on their vast social-media platforms. In 2007, on one of his video blogs, John Green, author of the best-selling “The Fault in Our Stars,” said he had a massive crush on Blume. When I emailed him to ask about Blume, he called me less than a minute after I hit “send.” Green describes “Forever . . . ” — about a loving, sexual relationship between two 17-year-olds — “as a hugely important book in the history of literature for teenagers.” It was not just that she actually wrote about sex, which was groundbreaking, but that she tried to reframe a cultural conception of it. “I have a radically feminist mother, and so I was always taught that sexuality was, you know, good,” Green said. “But there’s so many messages out there that sex is something that men do to women. It’s so hard not to internalize that. ‘Forever . . . ’ was a very different way of thinking about sex.”

In “Forever . . . ,” the two lovers are not caught or humiliated. The girl, Katherine, does not become pregnant, nor is she slut-shamed by her peers. Nothing happens, really, except that she and her boyfriend, Michael, eventually break up, with some rancor, none of it fatal, after Katherine meets someone else who seems very nice. “A contemporary book like that for young adults may well exist,” said Lizzie Skurnick, who wrote “Shelf Discovery,” a 2009 book of essays about young-adult literature. “But I haven’t seen it.” Carolyn Mackler, author of the young-adult novel “The Earth, My Butt, and Other Big Round Things,” said she thought about “Forever . . . ” as she sat down to write a scene for a new book in which a young man has sex for the first time. “She just wrote it organically, in a way that was true to her characters,” Mackler said. “Her novels made me want to write the most honest teenage characters I can.”