MAYBE it started with the Lethal Weapon series. Back in 1988, when the director Richard Donner was looking to fill the role of Mel Gibson's love interest in Lethal Weapon 2, he followed the formula laid out in every casting director's dust-covered handbook. He chose 20-year-old Patsy Kensit to play a smooth-skinned and naive young thing opposite the 32-year-old actor. But Patsy took a bullet, so when the time came to shoot Lethal Weapon 3 in 1991, Donner found himself playing matchmaker again. This time, though, he went with the unexpected: Rene Russo, a woman who was Gibson's equal in every way - including age. ''Dick likes the fact that she was in her 30s and not some ingenue,'' says Lauren Schuler-Donner, the director's wife and adviser on all three Lethal Weapons. After years of modelling and a string of so-so roles in so-so films (One Good Cop , Mr Destiny ), Russo found herself in one of the year's top-grossing movies. The director, Wolfgang Peterson, saw her Lethal performance and hand-picked her to star opposite Clint Eastwood in this summer's In The Line Of Fire - another potential blockbuster. So, at the age of 38, Russo now finds herself approaching Hollywood's elite group of A-list actresses. For a woman to hit her stride at that age in this town would have been unheard of a few years ago. And Russo is not alone. This summer, Sela Ward, 35,will get her first real crack at feature film stardom, opposite Harrison Ford in The Fugitive. Thirty-five-year-old Sharon Stone romances William Baldwin - a man six years her junior - in Sliver . Stone leads a select group of the most powerful and sought-after actresses in Hollywood, all in their 30s. There is Michelle Pfeiffer, 34; Annette Bening, 35; Geena Davis, 36; Demi Moore, 30; Jodie Foster, 30; Melanie Griffith, 36; Andie McDowell, 35, Ellen Barkin, 38; and Emma Thompson, 34. While twentysomething starlets certainly haven't disappeared from the screen - Julia Roberts, 26, is set to shoot The Pelican Brief later this year - women in their 30s have been quietly but dramatically taking over many of the choicest roles. Over the years, many powerful actresses have been able to hold on to their stardom past their 30th birthdays - there were Bette Davis, Audrey Hepburn and Joan Crawford, to name just a few. What makes the current crop of stars unique is that many of them,like Stone and Russo, didn't become household names until they reached their 30s. ''That you can still be made a star at Sharon Stone's age, well this is a first,'' says the casting director of Desperately Seeking Susan, Billy Hopkins. Why are we seeing this sudden mid-30s surge? One key factor behind the ''breakthrough'' is - no surprise - Hollywood's men. With the exception of Costner and Cruise, today's leading men (Harrison Ford, Michael Douglas, Arnold Schwarzenegger, Sean Connery, Clint Eastwood) are either well into their 40s, entering their 50s or, dare we say it, pushing 60. SO the big game in town has become lining up female co-stars whom the audience will buy as a love interest for these actors. ''When I go to the movies and see a 50-year-old male star making it with a 20-year-old, I always think to myself. 'In your dreams, buddy,','' says the screenwriter Callie Khouri, who won an Oscar for Thelma And Louise. ''It's just not something that most women will accept or believe - and that's part of the reason why the big change has come.'' In another era, the powers-that-be would not think twice about matching a recent college grad with an ageing stud. But today's studio heads don't want to do anything to offend the baby-boomer women, aged 20 to 45, who now make up the greatest part of film-going audiences. Geena Davis said recently: ''I've always felt that the industry could sustain very interesting, complicated parts for women.'' Her films, Thelma And Louise and A League Of Their Own, helped to showcase the trend. ''I'm happy the box office is high because that means more of these movies will be made.'' It also means women will start to wield more power in Hollywood. The current crop of thirtysomething actresses are learning that they can use their box-office muscle to get what they want. Along with being financially wise, the casting of the thirtysomething actress has become politically correct, a concern that packs quite a wallop in Hollywood. ''We owe Hillary Clinton a lot,'' says Denise Di Novi, producer of Heathers and the forthcoming all-female western, Outlaws . ''Her rise to prominence and the abortion-rights issue have created a huge resurgence of feminism in the Hollywood community and in general, and this is reflected in the studio's choice of materials and stars.'' Even script descriptions are being altered. Until a couple of years ago, the ages of leading ladies in submitted projects almost always read ''Jane Doe, twentyish'', or ''Jane Doe, a lovely young thing''. Now, say the script-scanners, the age descriptions are slanted towards ''late 20s'' (which can be played by the 30-plus actresses with box office clout), ''30s'', or they are left entirely to the imagination. Nevertheless, the problems that have traditionally plagued women past the age of 30 are still there: they have just been moved back a decade. One casting director proved this point when he was asked to pin a description next to the following actresses. Sharon Stone? ''Hot,'' he said breathlessly. Geena Davis? ''Hot.'' Meryl Streep? ''Old.'' At 51, Harrison Ford can still compete for roles with Alec Baldwin, 35. But the difference between Stone at 35 and Meryl Streep at 43 is the difference between a sexy, dangerous beauty (Basic Instinct) and an ageing, bitchy beauty (Death Becomes Her). Thirtysomething is the ideal age bracket for actresses, but pass 40 and you are again on thin ice. The couple in Indecent Proposal, for example, were originally written much older. The agency CAA fought to get Cher, 47, cast. ''But,'' says one source, ''Robert Redford did not want to do it with a fortyish actress. He wanted someone on the golden side of 35. And that was that.'' Demi Moore, 30, was cast.