When the September issues of fashion magazines — typically their biggest of the year, ad-wise — hit newsstands this month, the three most prestigious ones will feature actresses in their 40s on the cover.

Vogue has booked Halle Berry, who turns 44 on Saturday; Harper’s Bazaar will have 41-year-old Jennifer Aniston, promoting “The Switch”; and Elle has landed 42-year-old Julia Roberts, promoting her new film, “Eat, Pray, Love.”

Putting these actresses on the cover of arguably the most important issue of the year sends a message that though we live in a youth-obsessed culture, there’s still something to be said for the enduring appeal of women who have been in the public eye for nearly 20 years.

Laura Brown, projects/features director at Harper’s Bazaar, pointed out that the average reader of the magazine is in their late 30s — and that the readers view women like Aniston as “aspirational.”

“They’re cool, fashionable, interesting, compelling — they have something to say,” Brown said. “I love that they’ve grown into their style. One of the things about getting older is you do grow into your sense of self. You don’t look victim-y anymore.”

Lesley Jane Seymour, editor of More magazine, which is targeted to women in their 40s and older, echoed that sentiment.

“They’re the ones with real style, real staying power, real beauty,” said Seymour, who previously edited Marie Claire. “As the American population continues to grow older, everyone can relate better to a woman with a little wear on her tires.”

Plus, she added, “Who is there with any kind of real style or longevity in their 30s or 20s right now? Britney Spears? Kim Kardashian? These are flashes in the pan. Many are shallow reality stars like Snooki. Style icon? Um, talk to me in a year. Frankly, it’s here today, gone tomorrow. Lindsay Lohan? What’s to look up to?”

The statistics for several magazines bear her out. The average age of Vogue cover models in the past year is 34.5; Harper’s Bazaar, 32.5; Elle, 31.6; InStyle, 34.2; and W, 34.9. Harper’s had the biggest range, putting 17-year-old Miley Cyrus on its February cover and 47-year-old Demi Moore out front in April. (Moore also graced the cover of the December issue of W; she is the oldest cover model for those five magazines.)

Data released Monday by the Audit Bureau of Circulations showed that single-copy sales of magazines dropped by 5.6% in the most recent six-month period measured. So it’s ever more important for editors to select cover models who will appeal to people buying mags at newsstands, grocery stores and airports, where, of course, they pay full price. And in that context, older women can seem a safer bet, appealing to a wider audience — and a wider magazine-buying audience — than, say, a younger reality show star, singer or actress.

Indeed, it might seem as if the “Twilight” films have taken over the zeitgeist, but those movies’ twentysomething stars don’t come close to the star power of older actresses. At 46, Sandra Bullock is now the highest-earning woman in Hollywood, according to Forbes; she made $56 million last year with “The Proposal” and “The Blind Side.” Rounding out the top five were two actresses in their 30s, Reese Witherspoon at No. 2 and Cameron Diaz at No. 3, and two in their 40s, Aniston (No. 4) and Sarah Jessica Parker (No. 5).

It remains to be seen whether Kristen Stewart can open a non-“Twilight” movie, but in the meantime, films starring the likes of Parker (“Sex and the City 2”), 49-year-old Julianne Moore and 52-year-old Annette Bening (both from “The Kids Are All Right”), 35-year-old Angelina Jolie (“Salt”), and 40-year-old Tina Fey (“Date Night”) were box office draws this summer.

Outside of Stewart, no actress in her 20s has held a significant role in a box office hit this summer. Megan Fox in “Jonah Hex” is the prime example of a twentysomething struggling for audience.

Indeed, the age of a cover subject seems to show that older often can be better. On InStyle‘s covers, 40-year-old Gwen Stefani outsold 25-year-olds Scarlett Johansson and Leona Lewis (648,000, 579,000, and 610,000, respectively) — and all three were outsold by Jennifer Lopez, who was 40 when she was on the cover in September and sold a whopping 853,000 copies (though September, as mentioned, usually is the biggest month).

But not every over-40 woman is a sure thing, of course. Whitney Houston, then 46, showed up on January’s cover and sold a relatively paltry 408,000 copies.

At Elle, when Parker, then 44, was on the cover in December, she outsold Lopez on the cover from February (285,000 copies vs. 161,000 copies on newsstands) and 20-year-old Stewart’s June cover (207,000 copies). Vogue also had good luck with Parker, whose May cover sold nearly 325,000 copies on the newsstand. Parker outsold June’s cover with 22-year-old Blake Lively on the cover (248,000) and December’s with Cate Blanchett, then 40, out front.

“I will put anybody of any age on the cover,” Bazaar‘s Brown said. “I never think of someone’s age when I’m booking a cover.” Bazaar has been relatively steady on the newsstand, with its youngest cover model, Cyrus, barely outselling its oldest, Moore (126,000 vs. 114,000 copies).

Elle editor-in-chief Robbie Myers likewise was adamant that a cover subject’s age has no bearing on whether the magazine books her. “We don’t look at a subject’s age when we decide to put her on the cover,” she said. “It’s a question of a certain kind of chic, what the project is and how excited we think the buyer will be about reading about someone at a particular moment.”

But it’s perhaps not surprising that Myers chose Roberts, whose “Eat Pray Love,” which opens Friday, seems to speak to a certain recession-era mentality of self-discovery and authenticity. And Roberts herself has a carefully cultivated persona of keeping herself above the tabloid fray, putting her children first and choosing her projects carefully.

Jon Penn, a magazine consultant and president of the media and entertainment division of Penn, Schoen & Berland who works clients (including several from Conde Nast) to do market research on magazine covers, said it’s not surprising that editors are choosing seemingly approachable, down-to-earth actresses like Roberts for their covers.

“In the post-recession era, consumer values have changed at the newsstand,” Penn said. “Instead of seeking out escapism into the lives of over-the-top and often out-of-bounds celebrities, consumers are drawn to the authentic, down-to-earth and relatable. We are in a period of reflection where self-improvement, not self-indulgence, is a new language at the newsstand.”

There’s also less of a chance that a cover model of Aniston’s or Roberts’ ilk is going to end up scandalously in the tabloids (though Bullock probably would beg to differ). Brown noted that working with these types of actresses in many ways makes the magazine’s job easier.

“They give you just enough for the magazine,” Brown said. “They know how to conduct an interview. Sarah Jessica Parker would give you the impression that she would sit with you all day.”

Perhaps the more salient question, then, is who will be the Parkers and Robertses of tomorrow’s magazine covers. Of the cover models on Vogue, Harper’s Bazaar, Elle, and W in the past year, no one appeared on all four. But Parker, Blanchett, Moore, and Aniston each appeared on two, along with 37-year-old Gwyneth Paltrow and 35-year-old Kate Moss. The only woman under 30 to appear on two of those magazine’s covers was 22-year-old Rihanna, who was on the cover of Elle in July and W in February.

“The young actresses are fascinating in their own way,” Brown said, “but they need to be put in the wine barrel a bit longer.”