‘It felt like she was writing for me.” It is a sentiment I hear over and over again, talking to women in their 40s and 50s about the American writer Judy Blume, one of the world’s bestselling authors, who started writing young adult fiction in the 1970s, when that genre was still in its infancy.

As a 12-year-old growing up in Bath, my Judy moment, discovering her novel Are You There God? It’s Me, Margaret, due to be released as a film in the UK on 19 May, was revelatory. There I was, in the stonewashed denim jacket and matching jeans that my mum had bought me from C&A, feeling like the only girl in school who might never grow a decent pair of boobs or start her periods. (At 46, I’m still waiting for the boobs.) Then along came Margaret. Never mind that she was living more than a decade earlier, when they used alarming-sounding contraptions called sanitary belts; she was feeling the same anxieties, and everything she felt was right there on the page.

“I loved her immediately and for ever because she was writing about what it was really like to be a girl, worrying about bras, periods and crushes,” says the novelist Emily Barr.

Until she “found Judy”, Barr had been existing on a diet of Malory Towers, Enid Blyton’s wholesome boarding school novels. “Those girls did get older but they basically stayed like children,” she says. “Daryl Rivers definitely didn’t ever have a period.” Then Barr found Margaret and her friends, chanting “I must, I must improve my bust” in the hope they would grow to fill their new bras, and everything changed.

“Not only did these new girls have periods, they agonised about it,” Barr says. “It was trailblazing.”

When Barr started writing young adult fiction herself, she says the most common feedback from her editor was to “let the reader in” a bit more. “With [this genre] you really need to get into the inner life of the character,” she explains. “I still think about Judy when I’m doing that.”

Blume, whose 29 books have sold almost 90 million copies, has never faded away, but right now, at the age of 85, she is having something of a renaissance. As well as the movie, in cinemas next month Amazon Prime is releasing Judy Blume Forever, a documentary exploring the writer’s own journey from “fearful imaginative child to storytelling pioneer”.

Directors Davina Pardo and Leah Wolchok also focus on her fight against censorship, with her books still banned in schools and libraries in some US states for daring to talk about puberty and sex.

All the grown-up Judy fans I speak to are quick to say they will be watching what they call “The Margaret Movie”. “Oh my God, I’ll be there on the first day!” Barr enthuses. But I wonder whether the cinema will be entirely full of women of a similar age to me, reliving their adolescent angst in the dark and feeling relieved they can laugh about it all now. Will today’s tweens see the appeal?

Kelly Fremon Craig, who wrote the screenplay and directed the movie, is adamant that these books are “timeless”. Finding Blume at the age of 11, before which she admits she was “basically allergic to books”, she felt the author had a “direct line to all of my private thoughts and feelings”.

It was a huge relief, she recalls. “I so very much appreciated the way she told the truth about adolescence and didn’t hold back on the details”. She argues that some of those details may be very different now – but the feelings are still the same.

Young adult novelist Lisa Williamson read all Blume’s books in her youth, and loved the fact that “there was no pretence; they felt utterly real”. Williamson’s first novel, The Art of Being Normal, was widely praised for its sensitive portrayal of life as a transgender teenager, and she says rereading Blume’s books is “a nice reminder of how far we’ve moved on, that young queer people can easily find images of themselves in books”.

Yet she agrees with Fremon Craig that some aspects of young people’s inner lives don’t change. “I think it’s a misconception that they are maturing faster now. Yes, a lot of people might be getting their period and going through puberty earlier, but I don’t think emotions are changing.”

Michael Reiss, professor of science education at University College London, says Blume’s books filled a vacuum in the 70s and 80s: although young people were taught about the factual, biological aspects of periods, puberty and sex, “what you didn’t get was setting it in any sort of personal context”.

“Thirty or 40 years ago, teenage boys would eventually pluck up the courage to buy a copy of Playboy and pass it round,” he adds. “Now the majority of young teenagers will have seen some really unpleasant porn images.”

Helen McGarry, who is in her 40s and works on the school improvement team of an academy trust, remembers how mortified she was when the local librarian asked her “quite prudish” mother to give permission for her to take out Forever. This is Blume’s 1975 novel about a young couple having sex for the first time, and her legions of older fans vividly remember it being passed around under desk lids when they were growing up. Many also admit they still snigger when they hear the name Ralph, which was what Michael, the lead character, had christened his penis.

“My mum picked up the book and started flicking through it and seeing all these passages and she was wide-eyed,” she says. “In the end she said: ‘Well, as long as you’re sensible with it.’ We never mentioned it again.”

My 12-year-old daughter, Iris, has been deluged with lessons on puberty, periods and sex since starting secondary school. She initially raised an eyebrow when I bought her a copy of Are You There God? It’s Me, Margaret, utterly baffled that anybody would choose to write to God about breasts and periods. But when she saw that God wasn’t really the point, she was hooked.

I tell her that sex education was a very different and patchy thing when I was young, and that very few people discussed this stuff with their mums. I ask her whether all her sex and social education lessons mean that she doesn’t need books in the same way that we did. She looks at me in surprise. “Of course not,” she says. “We might be learning about it, but nobody discusses it. I don’t really talk about periods or bras with my friends.”

Apparently, even now, it’s rather nice to open a book and suddenly feel understood.

• This article was amended on 13 April 2023. An earlier version said that the genre of young adult fiction had not existed in the 1970s, when it had been conceived in about the 1960s.