Last Halloween, a five-year-old girl showed up at my doorstep wearing a tube top, miniskirt, platform shoes and eye shadow. The outfit projected a rather tawdry sexuality. "I'm a Bratz!" the tot piped up proudly, a look-alike doll clutched in her chubby fist. I had a dizzying flashback to an image of a child prostitute I had seen in Cambodia, in a disturbingly similar outfit.

I was startled, but perhaps I should not have been. In recent years, the sexy little girl has become insistently present in the media – from 15-year-old Miley Cyrus photographed draped in a sheet for Vanity Fair to websites "counting down" to the day that child stars, such as Emma Watson, reach the age of consent. And, of course, there was Britney Spears, aged 16, prancing around in school uniform and pigtails in her first music video. Their allure is that of "Lolita" – very young and very provocative.

Lolita has become shorthand for a prematurely sexual girl – one who, by legal definition, is outlawed from sexual activity. The Lolitas of our time are defined as deliberate sexual provocateurs, luring adults into wickedness and transgressing moral and legal codes. But the original Lolita – the 12-year-old protagonist of Vladimir Nabokov's novel – was rather different; a powerless victim of her predatory stepfather.

Like many pre-adolescent girls, she is sexually curious, but has no control over the abusive relationship. Yet it is as though the very fact of her sexuality has made her into a fantasy, rather than the novel's sexually abused and tragic figure. She is eagerly invoked in the media as a sign of how licentious little girls can be. "Bring back school uniforms for little Lolitas!" demands the Daily Telegraph in an article condemning contemporary sexy schoolgirl fashions, while Tokyo's Daily Yomiuri refers to "the Lolita-like sex appeal" of preteen Japanese anime characters.

Increasingly, young girls are seen as valid participants in a public culture of sex. In some ways, this is not new: in the 1933 film Polly Tix in Washington, four-year-old Shirley Temple played a pint-sized prostitute. And it's striking that the role of child prostitute was the springboard for the careers of many of our sex godesses: not just Temple, but also the 14-year-old Jodie Foster in Taxi Driver, 12-year-old Brooke Shields in Pretty Baby, and 13-year-old Penelope Cruz in a French soap, Série Rose. All are commentaries on child sexual exploitation, but the titillating representations positioned these actors as sex symbols and reinforced the link between girls' sexuality and sex work.

Yet in the middle part of the last century, our icons of female sexuality were Marilyn Monroe, 27, as Lorelei Lee in Gentlemen Prefer Blondes or Sophia Loren, 23, in Desire Under the Elms. Legally and physically adults, their much-admired bodies would not meet today's standards of sculpted muscularity and narrow-hipped leanness. The British model Twiggy is often cited for introducing the boyish, adolescent body type as a western feminine ideal. She was 16 when she started modelling in 1966 and by the late 80s the slender adolescent body had come to epitomise female beauty.

"A girl at the edge of puberty has a naturally hairless body that demands no shaving, waxing or chemicals . . . Her body is naturally small, supple and nothing if not youthful," observes sociologist Wendy Chapkis. The western ideal of female beauty, she writes, is defined by "eternal youth".

This emphasis on youthfulness has led to the use of very young girls as models in fashion and advertising, often in sexually suggestive contexts. Most catwalk models are between 14 and 19 – some, such as Maddison Gabriel, the official face of Australia's Gold Coast fashion week in 2007, are just 12.

Young girls are increasingly posed as sexual objects of the adult gaze, while numerous clothing ads feature women dressed as little girls, sucking on lollipops, kneeling, crouching or lying in positions of subordination. Witness the 20-year-old model Lily Cole, ribbons in her hair, clasping a teddy bear for French Playboy. Childishness is sexy, these messages seem to say. Ergo, children – especially little girls – are sexy.

The highly sexual poses imply they are "Lolitas" – knowledgeable, wanton, seductive. It sends a message that little girls should be viewed as sexy. The idea is that female sexuality is the province of youth. Writing in the New York Times, children's magazine editor Pilar Guzman observes, "The gap is diminishing between what's meant for children and what's intended for their elders."

It's called "kids getting older younger" – a marketing construct blurring the line between adults and children, especially with regard to sexuality. The problem is not with children, but with those who knowingly sell products with powerful sexual overtones to young girls, and with adults who then interpret girls' bodies as sexually available.

If these little girls can't feel sexual desire or understand much about it, why are we so obsessed with fetishising them? A possible answer is a backlash against feminism. Society has been forced to confront women as contenders in the social arena. This has generated resentment from men, as in Michael Noer's infamous 2006 column in Forbes, "Don't marry a career woman," in which he claimed that working women are more likely to cheat on their husbands. Little girls epitomise a patriarchal society's ideal of compliant, docile sexuality. In the media, girls are reduced to one-dimensional, wholly limited figurines.

But the motivation is also commercial. Cosmetics and fashion designers are finding ways to capture loyal consumers almost from day one. On the flip side, emphasising girlishness as desirable facilitates the multibillion-dollar sales of anti-aging cosmetics, creams and plastic surgery. Finally, there's the underground economy of little girls' sexuality: child sex trafficking and prostitution. According to the UN, sex trafficking is the fastest-growing area of organised crime.

I want my two young daughters – indeed, all girls – to grow up confident about finding and expressing sexual pleasure. But as a culture, we have few ways to represent or acknowledge children's sexuality, and we seem incapable of dealing with it outside the realm of sexual commodification and commerce. Sexual curiosity and even some experimentation are ordinary features of childhood. Realistic, strong, and non-exploitative representations of girls' sexuality would be a progressive social step, but images of girls posed and styled as objects of the erotic adult gaze can't be. They often employ the conventions of sex work, legitimising the use of young girls for prostitution and pornography.

I wish that Halloween costumes for little girls involving vinyl boots or corsets were just silly and fun. They may be, in contexts where girls are totally protected, safe from any misreading or violation. But I am not convinced such contexts exist. Instead we must create safe and supportive spaces for girls to understand their sexuality on their own terms and in their own time.

• This is an edited extract from The Lolita Effect by M G Durham, Duckworth, £8.99. To order a copy for £8.99 with free UK p&p, call the Guardian Book Service on 0870 836 0875, or go to www.theguardian.com/bookshop.

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