Women’s spending on apparel peaks at age 44 and then enters a long slump, according to a Goldman Sachs research report. The question is, why?

Why would a woman’s enthusiasm for fashion slow just as her household income is reaching its highest level and her career and social requirements are at their most demanding?

When Cynthia Weber-Cleary, InStyle magazine’s former fashion director, launched her “Age-Wise Style” blog at the magazine in 2014, she heard from readers that most marketing seemed obsessed with younger women. Fortysomething mothers with young children didn’t see themselves as matronly. Older women didn’t want to look dowdy at their children’s weddings.

Ms. Weber-Cleary spied a business opportunity. She found a business partner inStephanie Stahl, former head of marketing at leather-goods maker Coach.

The duo, based in New York City, recently launched  (pronounced Ap-reesh-she-ay), a retail site offering style advice and fashions aimed at women over 45. The site never mentions the A-word, or anything related to age. The site’s relationships with retailers enable users to click through to buy on the retailers’ sites, while Apprécier takes a slice of the revenue—a standard model for such sites.

“Age is where the opportunity is,” says Ms. Stahl, who discusses the age strategy with investors. But “the kiss of death is to be labeled as the site for 45 and over. The only place on our site that mentions age is our bios.” Ms. Stahl is 49. Ms. Weber-Cleary is 58.

Women don’t want to be stuck in a ghetto with conservative or dumbed-down fashions that assume they want to hide their knees. Brands at all price ranges, from St. John to Talbots to Chico’s, have struggled not to be identified with women of a certain age, which could translate as stuffy or boring. Forth and Towne, Gap’s attempt at the over-35 market, went out of business in 2007 after less than two years.

Apprécier offers up-to-date fashion features such as “It’s Pajama time. Yes, Really!” on the pajamas-as-streetwear trend, and profiles stylish women. There are links to apparel from 600 brands and 200 online retailers, from Tory Burch to Intermix. Prices range widely. Recently, the main “boutique” page was dominated by an $88 Madewell denim shirt, with a smaller highlight of $525 Golden Goose sneakers, at Farfetch, and $395 Rag and Bone track pants, at Net-a-Porter. Another page offered an $1,890 Carolina Herreratop.