By Jennifer Alfano,

No miniskirts after 40. Or long hair. Sleeveless after 50? Never! There are a lot of women who still believe in age-related fashion dictums, but in a world where everyone from nine to 90 wears jeans, what does “age appropriate” actually mean?

“I don’t think a number defines your age,” says designer Marissa Webb, who’s in her 30s. “Your lifestyle and energy do. I’ve met many women in their 50s who are in amazing shape and can pretty much pull off any outfit. What’s appropriate for one person can be very different for another.” Thirtysomething designer Veronica Swanson Beard concurs. “Dressing your age means feeling comfortable in your own skin and working with what you’ve got.”

In my case, now that I’m in my 40s, I find myself wanting to dress not necessarily younger but edgier. Classics that I once loved, like tailored bouclé jackets and basic LBDs, suddenly seem aging. I want to push my boundaries, to find silhouettes that are modern and clean and have a twist. I need a little funk. Not a lot. But enough to keep it all interesting.

Perhaps this is just a reaction to the clothes that designers are creating now, which happen to have a bit of edge and make me excited to get dressed in the morning. Sarah Burton at Alexander McQueen cuts perfect print dresses that work whether you’re 25 or 65; the tailored tuxedo pieces Alber Elbaz at Lanvin sent out for spring meet my fashion-forward desires; and almost anything from Phoebe Philo at Céline is a winning combo of cool and comfort. These designers are making grown-up clothes. They’re avant-garde yet polished. And that polish is key. Sure, I can wear the same jeans my daughter does, but then everything else needs to be elevated with, say, a Balmain blazer and a cashmere sweater from the Row. Maybe I’ll add a pair of pumps, but these days I find that only one “timeless” element really feels right. Though I like my leather pants from the Row, miniskirts, for me, are better left in the past. I see plenty of fortysomething women (Gwyneth Paltrow, et al.) wearing them and look-ing fabulous, but I’d rather rock a Dolce & Gabbana pencil skirt with a spiked stiletto any day. When it comes to handbags, the last thing I want is something traditional. I don’t want an It bag, but I do want something fun. (As I write this, I’m having a daytime-clutch moment; Céline and Bottega Veneta are my favorites.) Of course, if you’re in your 20s you can wear classic, because youth alone gives it that essential ironic touch. Plus, you can take the seriousness out of anything by mixing high with low. Just look at Alexa Chung, who dresses as if she mashed up her mother’s and grandmothers’ closets with her own and came out with ensembles that are winsome, sexy, and girlish. (Denim cutoffs with Chanel bouclé wool jackets, Valentino paired with Topshop—it all looks great. But on me not so much.) When can you begin to dress straight-up überclassic and make it beyond chic? I’d say your 70s. That’s when you want to be impeccable and understated. (Think Lee Radziwill and Deeda Blair.) Ladylike suits, ladylike bags—it all works.

Another important aspect of dressing your age is that once you hit your late 30s you want to refine and rely on a go-to look. I asked designer Jenni Kayne, 30, what she can wear today that she wouldn’t have worn when she was younger. Her answer: “A uniform. It takes maturity and confidence to define your style.” So true. But not always so easy.

Pictured above: 20s Gaia Repossi, 30s Jenni Kayne, 30s Marissa Webb, 40s Lucie de la Falaise, 50s Linda Fargo

While your uniform can evolve with your tastes, by your mid-30s you usually develop a sense of what best suits your body, your personality, and your lifestyle (and which is typically anything that makes you feel fantastic), and learn to resist the extraneous noise of other clothes and accessories that don’t fit those criteria. Like Kayne, I try to create a uniform that I can rely on. What always does it for me? J Brand and MiH jeans, simple lightweight cashmere crewnecks from Céline, or a beautiful silk blouse from Stella McCartney or Lanvin, with heels (again) from Céline or Alexander Wang. If the weather is cool enough, my outfit often revolves around my outerwear, and that’s where I will willingly plunk down extensive funds for a standout blazer or coat. My favorite is a leather-trimmed trench from Rag & Bone.

With age comes the opportunity to dress with devastating sophistication. For Swanson Beard, entering her 30s offers a chance to wear “tuxedos. There’s nothing sexier than a tailored tux and stilettos,” she says. “It’s so Helmut Newton ’70s!” For her newly 40 business partner and sister-in-law, Veronica Miele Beard, it’s jumpsuits. “I would have opted for the short, flirty dress when I was in my 20s, but I feel just as dressed up, confident, and comfortable in a well-fitted jumpsuit.” And for Webb, it’s shocking-red lipstick by day. “When I was younger, I felt as if I was still playing with my mother’s makeup. As I get more mature, I see it as another accessory, like shoes or a bright handbag.” Ask these women what is instantly aging and their answers vary. “For me, it’s anything too tight, too short, or too girly,” Kayne says. Swanson Beard’s no-nos are “too much makeup and too much jewelry.”

In the end, the key to successfully looking age appropriate and modern is paying attention to what feels right on you. Take your cue from women who do it well, like Kate Moss, Phoebe Philo, and Vera Wang. These are women who dress their age but still look innovative and, above all, like themselves.

In a world where everyone from nine to 90 wears jeans, what does “age appropriate” actually mean?

“It takes maturity and confidence to define your style,” says Jenni Kayne.So true. But not always so easy.

I see plenty of fortysomething women wearing miniskirts and looking fabulous.

For http://www.harpersbazaar.com April 25, 2013