Inside the first 10 minutes of the new film Book Club, Vivian (Jane Fonda) instructs everybody to get laid. Diane (a ne plus ultra Diane Keaton) grabs a man’s penis on a plane, Sharon (Candice Bergen) is diagnosed with a “real lethargic pussy” (her cat, but you get the picture) and Carol (Mary Steenburgen) delivers a knockout punchline to the suggestion that ageing vaginas are a subject worthy of Werner Herzog: “The Cave of Forgotten Dreams!” The four women hoot over large, misted glasses of white wine and the first of the film’s many lavish smörgåsbords.

I think I can speak for most people when I say that I prefer not to consider my parents’ sex lives. But Book Club’s evidently less repressed creators were inspired after sending copies of Fifty Shades of Grey to their mothers on Mother’s Day in 2012. They recognised the enthusiastic responses as an opportunity “to celebrate women at a stage in life when they don’t feel like they get the opportunity to see themselves on screen”, says Bill Holderman, who co-wrote and directed the film. He listened to the audio versions of the Fifty Shades books in the car, praying he wouldn’t “get into an accident or get pulled over”.

And so it was that EL James’s erotica series became the crux of a film about a group of accomplished women. Their 40-year friendship is anchored by monthly book-group meetings that inevitably turn to gossip and laments about their sex lives. Vivian, a hotelier, presents the first instalment to great resistance (more because of its lowbrow reputation than its kinkiness), but soon they are all sneakily reading the book and finding it extremely stimulating, despite James’s stilted prose. “Ohhhhh wowww,” Carol purrs as she reads while watering a plant. The camera cuts to the moisture meter stuck in the soil, its dial moving from dry to wet.

The power of older cinemagoers is increasingly well understood in UK cinema: the percentage of audiences who are 55 and over has jumped from 7.4% in 2009 to 12% in 2016. But a UK Film Council survey in 2011 showed that 60% of older female filmgoers were tired of seeing themselves portrayed on screen as “sexless grandmothers”, while recent statistics suggest that more than 40% of Americans aged between 65 and 80 are sexually active. Medical professionals urge the need for more discussion about the subject, particularly as people from every generation are prone to sexually transmitted infections. Book Club is, no pun intended, filling a hole.

There’s some precedent for the film’s portrayal of older women as sexual protagonists. In 1971, Harold and Maude depicted a 20-year-old boy falling in love with an eccentric octogenarian, a relationship consummated in a glorious scene that leaves Harold blowing bubbles in disbelief. Less absurd, although fantastic in a different way, is the oeuvre of Nancy Meyers, the great American formalist who has made a specialty of putting older women’s sex lives on screen. In Something’s Gotta Give (2003), Jack Nicholson’s ageing music mogul Harry has a heart attack while attempting to bed twentysomething Marin (Amanda Peet) and must recover at the home of her mother, Erica (Diane Keaton again). He and Erica fall in love and he renounces his dogged pursuit of younger women. “I’ve never seen a woman that old naked before!” he exclaims. Thanks to the blink-and-you’ll-miss-it nude scene, the audience don’t on this occasion, either.

There’s not much sex in Something’s Gotta Give compared with Meyers’ 2013 film It’s Complicated, in which Meryl Streep’s Jane ends up having an affair with her ex-husband, Jake (Alec Baldwin), while being pursued by her architect, Adam (Steve Martin). “People over 50 talking about sex and – yikes! – having it!” wrote Rolling Stone’s Peter Travers. “Women of a certain age won’t know which is sexier,” wrote the Washington Post’s Michael O’Sullivan. “That Jane is lusted over by two handsome suitors or that she’s finally getting her dream kitchen.” The rambunctious sex scenes between Streep and Baldwin are relatively explicit and garden sprinklers start spraying with impeccably rude timing. “The look of deep satisfaction on Baldwin’s face is hilarious and, for an American movie, shocking,” wrote the Boston Globe’s Wesley Morris. “When has a man made that face after sex with any woman, let alone as senior as Streep?”

This summer, the cast of Mamma Mia! will once again chew their way through the Aegean scenery in a sequel to the 2008 film. Here We Go Again flashes back to the earlier life of Donna (Meryl Streep again) to find out how she fared as a single mother bringing up her daughter, Sophie. But focusing the film on the younger Donna (played by Lily James) loses the radical middle-aged verve of the original, in which, despite Donna’s embarrassment at not knowing who Sophie’s father is (there are three candidates), her friends and daughter refuse to let her Donna be slut-shamed. The older women relish their enduring sexual appetites and ask Sophie whether she really wants to get married and settle down at 20 instead of going out and seeing the world.

It’s not surprising that these films are so rare: Holderman and Erin Simms, Book Club’s co-writer, had to battle to make the film. Studios repeatedly returned the same two notes: “‘Cast the movie younger,’ and ‘Don’t use the book Fifty Shades,’” says Holderman. “For us, they’re both fundamental to the movie and the idea – the conversation about our mothers and the age that they were.” He says he can’t put himself in the mindset of Hollywood executives, but acknowledges that they might have been squeamish about the idea of a film focused on women in their 60s, 70s and – in Fonda’s case, 80s – having sex.

“I do think [Hollywood] is male dominated and that [the subject of older women’s sex lives] is still, sadly, taboo in our culture – particularly in America,” says Holderman. He points out that Fifty Shades made BDSM mainstream, while older women’s sexuality is still a difficult subject to tackle. “I think women after a certain point are relegated and put out to pasture, and that is reflected in a lot of male studio executives’ mentalities. It’s too bad, but hopefully this movie will start to shift at least some people’s minds.”

Despite its raunchy inspiration, Book Club isn’t exactly Bad Grandma – the closest it comes to being explicit is Carol bedding Ed Begley Jr in the back of her car and emerging dishevelled. Maybe Old Girls’ Trip is a better description, the route to self-realisation paved with weapons-grade innuendo – Steenburgen’s character squirting cream cheese into mini peppers, endless winks about motorbike crankshafts – and age-appropriate jokes. When Diane goes out with a handsome pilot, her friends remind her that the last time she went on a date she wound up pregnant and married. “I don’t think that’s gonna happen this time,” says Fonda’s Vivian.

Thanks to some well-deployed literary references, we really do feel as if these women have been reading together for the past four decades. And Book Club’s mixture of high and low culture is admirably anti-snobbish, reflecting Simms’ and Holderman’s outlook as much as the classic high-low dynamic of romcom greats Meyers and Nora Ephron. Holderman takes the compliment. “Meyers is underrated because she is able to do something that is both commercially minded but sophisticated, and I think sometimes when that’s packaged in a comedy or romantic comedy setting people don’t take it seriously as an art form.”

He hopes that Book Club’s success will lead to more films about older women’s sex lives: in the US and Canada, it reached No 3 at the box office, behind superhero blockbusters Deadpool 2 and Avengers: Infinity War. In a video posted to Fonda’s Twitter account, the four principles of Book Club challenged Deadpool’s Ryan Reynolds to a showdown. “You’re not the only one who kills in a tight little red outfit,” she winks. “Or out of one,” adds Steenburgen. Keaton steps in: “Mary, didn’t you play his mother once?” She did, in The Proposal (2009).

Unsurprisingly, Holderman would love to do a sequel, just to spend more time with the actors. But, until then, it has had the desired effect. “Both our moms came to the premiere,” he says, “and I think both of them were thrilled and a bit inspired. That was kind of a dream scenario for us.”