YOUNG AT HEART

I was inspecting a rack of knit tops at a Century City boutique when two men walked in, undoubtedly seeking gifts for women in their lives, clearly on unfamiliar terrain.

“What age are your shoppers here?” one of the men asked a young saleswoman.

“21 . . . 22,” she answered.

At the time I was nearing 40. I felt a flicker of sheepishness-- You mean, I’m not supposed to be here? --followed by a surge of confidence. Hey, look again. Your clientele goes through middle age. I hadn’t made a mistake. The clerk had.

Check out the 40-year-old psychologist shopping the Macy’s junior department for low-slung cargo pants--to show off her pierced belly button--and the 51-year-old actress picking up short skirts at Rampage, that outpost of 20-something fashion.

At the top of the 21st century, the long-accepted fashion catechism has been vanquished. You can wear white in the winter and leather in the summer. Vintage clothing is so popular that it’s difficult to tell 1968 from 1998. Anarchy reigns at the hemline. And now, the most rigid of rules is crumbling: How old you are determines how you dress.

If the skirt fits, you can wear it.

What does “older” look like anyway? If, as a fashion magazine announced several years ago, 40 is the new 30, does that mean that 50 looks like 40? Does 60 look like 50? Certainly the image of the middle-aged woman and how she dresses has been renovated by contemporary Hollywood. Consider 41-year-old Madonna in skintight hip-huggers and 55-year-old Goldie Hawn in, well, everything she owns. Kim Cattrall, 44, plays the skimpily dressed Samantha in the HBO show “Sex and the City.” She’s the patron saint of all her female peers who believe that wearing clothes should continue to be an adventure in middle age. And if Diana, Princess of Wales, had lived into her 40s and 50s, she would have worn whatever looked sexy.

Of course, Hollywood skews the curve since celebrities have the money and time to sculpt themselves to ageless perfection through rigorous dieting, exercise and plastic surgery. But women in general have been reveling in the overthrow of fashion canons.

“We’re such a lucky generation,” says one 51-year-old size-6 actress who shops at Rampage. “Look at our mothers. They were matrons by the time they were 40. Now you don’t have to get dumpy and you don’t have to get dowdy. And you don’t have to be outrageous and grotesque either.”

Not only does “older” not look that old, it doesn’t feel that old either. “When I’m dressing just for me, I don’t see myself as much past my 20s,” muses the actress, who nonetheless won’t let her name be used because Hollywood marginalizes women over 30. “When I dress for my daughter’s college graduation, I will have to remind myself to dress like a mom. But not much like a mom.”

In 1957’s “An Affair to Remember,” Deborah Kerr, who was beautiful and funny and clever, dressed like a dowdy matron even as she was carrying on a romance with the dapper Cary Grant. And she was only 35.

These days, the role model for middle age looks neither like Deborah Kerr nor Mae West. It looks more like a cross between Sela Ward, the sultry 44-year-old star of the TV show “Once and Again,” and Katie Couric, the 44-year-old host of NBC’s “Today Show” who has been known to flex her trainer-enhanced biceps on morning TV.

Exercise, weightlifting, yoga and tofu have created legions of women, middle-aged and older, who never stopped following cutting-edge fashion and have the bodies to wear it. In choosing clothes, they hew to the only remaining universal benchmarks American women use in judging clothes: Does my stomach stick out, and does my butt look too big?

“I was just sitting here in Azzedine Alaia when you called,” says Franoise Kirkland, a partner in her husband Douglas Kirkland’s business as a celebrity and fashion photographer. At 57, she has a closet full of tight-fitting Dolce & Gabbana in addition to Alaia, known for his girdle-like knits.

With her long, dark hair and armfuls of chunky silver bangles, she dresses much the way she did in her 20s and 30s. Well, she wouldn’t poke a hole in a tablecloth and wear it as a skirt, which she did 30 years ago when she traveled to Portugal and her luggage was lost.

“When you’re younger, you can take three bandannas and make a halter top, and it looks great,” she says. “When you get older, you have to be more careful. You can wear the same style of clothes, but they have to fit perfectly.”

Some women tried to follow rules connected to age. “A couple of years ago I thought it meant cutting all my hair off,” says attorney Cynthia McClain-Hill, 43, who runs her own downtown law firm. “And I’m kind of annoyed I did it. So I’m letting it grow back.”

Women who throw out the guidelines about dressing their age still make some adjustments. One 40-year-old, preparing for a date, assessed herself in a sexy backless turtleneck sweater and decided she needed to wear a bra. “At 25, I wouldn’t have,” she says. Author Nancy Griffin, 49, possessed of legs as long as a giraffe’s, wears short skirts but prefers them now with stockings for dressing up at night.

Ever since I was freed of my Catholic high school uniform, I have embraced only a few rules of dressing: no cropped tops at work, and always wear shoes. At 45, I never wear pantyhose (unless it’s winter and I’m in Chicago) and I don’t subscribe to the fashion magazine adage that if you wore a trend the first time around you’re too old to wear it the second time around. I bought my first Diane Von Furstenberg wrap dress when I was 22. I bought my second at 42.

However, there are times when I stand in front of the mirror in a low-cut black knit camisole, appraise myself and wonder: Do I look like a hot chick or do I look like a road-worn floozy who, in the movies, would be played by Valerie Perrine? And--here’s a scary thought--can I tell the difference anymore?

Los Angeles, predictably, is a perfect incubator for the new ageless dressing and some corollary excesses. Few outfits are shocking. At the Tracey Ross boutique, purveyor of Chloe and other high-end designers, there are 50-year-old customers who look like they’re in their 30s, and teenagers who look like they’re in their 20s. “I had a 12-year-old in here the other day,” says store manager Karen Zambos. “She looked 18.”

But store owner Ross, who will be 40 this summer, says there are certain items that are just too wild, too young for the over-40 sisterhood--such as hip-huggers with a dangerously low rise. “Even though we have the body for them, I don’t think we should be wearing them,” she says.

This is the same Tracey Ross who showed up at her store’s 10th anniversary party last fall wearing a leather skirt, skimpy tank and an array of belts and chains. “I want to kill myself, I looked like such a fashion victim,” she says now. “I love the T-shirt and the skirt, but I’m wearing a Sid Vicious dog collar around my neck.”

In an alcove of Ross’ store on a sunny Saturday, Brenda Fulmer, 60, shopping with her daughter, Catherine, 20, is unworried about going too far. After all, this is the woman who bought studded Versace jeans in Italy under the guidance of her daughter, a fashion design student. “I wore them last Sunday night with high heels to a restaurant,” says Fulmer, a real estate broker in Palm Springs, with a pleased smile.

Fulmer, at 60, is something of a maverick in her peer group. But increasingly she has company. At the Neiman Marcus store in Beverly Hills, there is evidence of a rift in the elegantly dressed over-50 crowd. “There are still very much the Ladies Who Lunch who will come in in their suits and have their handbags and their slingbacks,” says John Martens, vice president and general manager of the Neiman Marcus store in Beverly Hills. “On the other hand, there is another group that doesn’t wear stockings; they wear sandals, and they wore mules last year.” Last year they wore python, this season it’s tight leather Gucci pants.

“In times gone by, there was a less sexy look,” says Martens, who has presided over the Beverly Hills store for 21 years. “If a woman of 50 wore a slinky Versace, people might look at her and wonder. But today they would say she looks great--and not because she looks great for her age, but because she just looks great.”

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On Alexandra: Giorgio Armani striped top, $850, at Giorgio Armani, Beverly Hills, with Dolce & Gabbana iridescent skirt, $288, at Dolce & Gabbana, Beverly Hills. Jimmy Choo shoes. On Viorica: Giorgio Armani ruffled jacket, $2,425, at Giorgio Armani, Beverly Hills, over Catherine Malandrino striped pink blouse, $198, at Catherine Malandrino, Sunset Plaza. Emporio Armani chiffon skirt, $168, at Emporio Armani, Beverly Hills. Jimmy Choo shoes.

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On Francoise: St. Vincent “Dior” silk dress, $370, at Sage Boutique, Los Angeles. On Nancy: Dolce & Gabbana silk-chiffon blouse, $655, at Dolce & Gabbana, Beverly Hills.

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On Morgan: St. Vincent micro-jersey top with gold eyelet, $102, at Sage Boutique, Los Angeles. Emporio Armani silk skirt with gathered waist, $298, at Emporio Armani boutique, Beverly Hills. Adriana Caras shoes. On Nancy: St. Vincent asymmetrical micro-jersey dress, $170, at Sage Boutique, Los Angeles. Giorgio Armani shoes.

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On Diana: Sold Apparel chocolate leather blazer, $1,495, at Traffic, Los Angeles. Earl Jean low-rise jeans in Vintage, $120, at Earl Jean boutique, Los Angeles. Adriana Caras sandals. On Karine: Sold Apparel leather dress with shoulder paillette, $1,400, at Traffic, Los Angeles. Adriana Caras sandals.

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On Kelly: St. Vincent one-shoulder top with antique rhinestone brooch, $106, at Sage

Boutique, Los Angeles. St. Vincent stretch poplin knee pants, $141, at Sage Boutique, Los Angeles. St. Vincent silk and leather belt. On Sue: St. Vincent scarf-neck inkblot top in stretch silk, $187, at Sage Boutique, Los Angeles. Catherine Malandrino pants with tie waist, $253, at Catherine Malandrino, Sunset Plaza.

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On Cecilia: Catherine Malandrino strapless dress, $297, at Catherine Malandrino, Sunset Plaza. Giorgio Armani shoes. On Onna: Armani Collezioni dress, $795, at Giorgio Armani, Beverly Hills.

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On Cynthia: Alberta Ferretti dress, from a selection at Neiman Marcus, Beverly Hills. Darcy Miro sterling silver bracelet. Giorgio Armani shoes. On Marysol: Alberta Ferretti dress, $2,590, at Little Italy, New York. Jimmy Choo shoes.

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Produced by Lisa Thackaberry; stylist: Magda Berliner; stylist assistant: Andrew Salazar; hair: Ken Paves/Profile and Dwayne Ross/Profile for Dionicia; makeup: Beth Katz; location, page 24: Johnie’s Broiler in Downey