When Sophia Loren turned 72 three years ago, she celebrated by posing semi-naked for a Pirelli calendar alongside a handful of women less than half her age. Wearing a silk negligée, she draped herself languorously under a bedsheet with her head tilted sensually upwards in a suggestion of ecstatic abandon. A Vatican aide was moved to comment that, if the Pope ever changed his stance on human cloning, it would be because of Loren.

In the same year, Brigitte Bardot was photographed on her way to a meeting with Nicolas Sarkozy, then French interior minister, to discuss cruelty against Canadian seals. Bardot was dressed in black and limping on crutches because of a nagging hip complaint. According to one mid-market tabloid, her appearance was marked by "sagging jowls covered in heavy make-up and a wiry, unkempt patch of greying hair".

The comparison might seem ungracious, but Bardot has the misfortune of being almost exactly the same age as her fellow actress: this month they will both turn 75 within days of each other. These two cinematic legends of the 1960s have chosen to age in very different ways, provoking very different public reactions. Their forthcoming birthdays prompt the inevitable question: how should an icon best grow old?

Dennis Nothdurft, curator of the Fashion and Textile Museum in London, believes it is an almost impossible conundrum. "The problem with being an icon is that it precludes them ever changing," he says. "We create icons, we associate them with a period of time, and then we want them to stay in that period."

And it is true that we do not expect our sex symbols to age. We want them to remain youthful and pouting at their most delectable, like insects in amber. If they do have the temerity to grow older, we expect them to be like Loren and segue seamlessly from sexpot youthfulness to granny glamour. We find it more difficult to fathom when, like Bardot, they choose to retire from public life, set up an animal welfare foundation and refuse to care about the creeping onset of their decrepitude.

"Brigitte Bardot has bowed out of the limelight, has refused plastic surgery and hasn't tried to maintain her looks," says Nothdurft. "That's quite refreshing because it's realistic. It is ageing on her own terms, not buying into the system, whereas Sophia Loren has completely embraced the idea of being attractive and sexual as an older woman, and she looks fantastic."

In their heyday Bardot and Loren came to epitomise the dual archetypes of fame and sexual power. Bardot, the star of New Wave French cinema who made her name in Roger Vadim's 1956 film And God Created Woman, was the blonde sex kitten whose natural femininity ushered in a new era of sexual authenticity. Loren, a former beauty pageant contestant from Naples who won a 1960 Academy Award for Two Women, was the brunette studio star who seeped stylised glamour from every pore.

"It was almost a cliché of the iconic brunette and the blonde stereotype," says Rebecca Lowthorpe, fashion features director of Elle magazine. "Bardot was more of a sexpot or bunny girl, while Loren was more of a sultry sophisticate. Both epitomised their nations' stereotypical style: the French nymphette sex toy and the Italian woman with full-on, big-haired glamour."

But their impact went deeper than this. In the 1960s, when the straitlaced morality of the previous decade was being overturned by the dawn of sexual freedom, Loren and Bardot seemed to embody a new kind of woman who crossed traditional boundaries. Loren scandalously married the divorced film producer Carlo Ponti at a time when Italy did not recognise divorce. She allegedly went on to have affairs with JFK, Peter Sellers and Omar Sharif.

The four-times-married Bardot, with her on-screen sexuality and off-screen promiscuity, prompted the feminist existentialist Simone de Beauvoir to write in 1962 that the actress was "as much hunter as she is prey… Bardot is as important an export [to France] as Renault automobiles." Bardot's sexual allure was thus not simply a ploy to ensnare men, but rather a challenge to other women. She was unapologetic about her sexuality: it seemed to be, like so much else about her, entirely natural.

According to the art dealer James Hyman, who is showing an exhibition of Bardot photographs at his London gallery to coincide with her birthday, Bardot was the antithesis of the manufactured film star of a previous age. "She was natural, she went barefoot, she didn't brush her hair, she wore no makeup, she wore [flat-soled] pumps because she trained as a ballet dancer. It's that image of freedom, exuberance and youth. She stood up as being authentic, instinctive, free. For women, it was a feminist thing; it was about women behaving as men did, taking lovers and having affairs."

Part of Bardot's enduring influence was also attributable to the emergence of the paparazzo. "She was very, very savvy and she knew the importance of press reporters and paparazzi," says Hyman. "It was the paparazzi who made her a star and she was complicit with that. They were not staged studio shots, but they catered for an age that wanted images of stars being more candid."

In contrast to Bardot's laissez-faire attitude, Loren was always careful of the image she presented of herself and, in many ways, this difference of approach is reflected in their individual attitudes towards the ageing process. Michael Winner, the restaurant critic and former film director who is a close friend of Loren, says that she is "very proud of her appearance" and "always very conscious, actually, of being photographed by members of the public, in case they got that one picture where she doesn't look any good, which can happen to all of us."

In 1979, when Winner was directing Loren in Firepower, he remembers getting a phone call from the actress some weeks after filming had stopped. "She asked: 'Have you got Paris Match?' I said: 'No, I don't get Paris Match, what's in it?' She said: 'There are 10 pages of me sunbathing topless in Antigua, do you think I should sue?' I said: 'Well Sophia, what do your breasts look like?' She said: 'Very good, actually,' and I said: 'Well, in that case, send them a thank-you note.'"

Whereas Bardot made a forte of appearing as natural as possible, Loren cultivated the image she wished to present of herself. This fundamental distinction was greatly magnified once their career paths began to diverge.

After appearing in more than 50 films from the age of 18, Bardot announced her retirement just before her 40th birthday. Choosing to devote herself to animal rights, she became a vegetarian and in 1986 established the Brigitte Bardot Foundation for the welfare and protection of animals in St Tropez.

In recent years she has gained a reputation for eccentricity and outspokenness. In 2008 Bardot wrote an open letter to the US Republican vice-presidential nominee, Sarah Palin, accusing her of being "a disgrace to women", and since 1997 she has been convicted five times of inciting racial hatred after comments made about the "Islamisation of France".

Loren, on the other hand, continues to work as an actress and is currently filming the musical Nine alongside Daniel Day-Lewis. She has famously claimed that her physique is a consequence of a daily diet of pasta. "Everything you see," she once said, "I owe to spaghetti."

"Sophia Loren never changed her career," says Hyman. "There is the occasional unpleasant newspaper article about how Bardot has let herself go, but the thing is that Bardot stepped away from the life she had as a young woman. It [her appearance] symbolises her turning her back on her earlier life. Loren is still engaging with her earlier life; she is still a star."

Winner puts it more succinctly: "Sophia is a great professional. She keeps working and she knows that, because of that, you'd better keep looking lovely. Brigitte Bardot is into saving cats. You don't need to look beautiful to pick up stray cats."

Both choices, in their own way, are admirable. Bardot, a woman of immense fame and beauty in her youth, has deliberately chosen to step outside the limelight and chart her life along a different course. Her charm has always been her naturalness: whether as a 19-year-old wearing a bikini on the beach at Cannes or as a woman in her 70s who is not afraid of displaying her wrinkle lines in public.

Loren, by contrast, defies any thought of retirement and continues to preserve her appearance and image in order to carry on working as she has always done. "The broader point with Sophia Loren and Brigitte Bardot is that it depends on the individual woman's view of how she wants to look as she ages," says Dr Nick Lowe, consultant dermatologist at the Cranley Clinic in London. "They've probably had two different lifestyles."

It is a simple point but a crucial one. Loren and Bardot might be the same age and come from similar backgrounds, but they have grown into two different women who have chosen two different lives and who feel no need to explain themselves. At 75, they are making their own rules. And that, perhaps, is the most iconic thing about them both.