Last year, 30-something friends Andria Lo and Valerie Luu released “Chinatown Pretty,” a photo book that lovingly captures the fashion sense of advanced-age Chinatown residents in cities including Oakland, New York and Chicago.

Also last year, Hsu Hsiu-e, 84 and Chang Wan-ji, 83—a married couple who own a laundromat in Taiwan—became global social media stars thanks to their Instagram account, @wantshowasyoung. The pair pose in compelling outfits styled from clothes their laundromat customers have left behind. The account is now up to over 654,000 followers and the pair was recently named the ambassadors for Taipei Fashion Week.

Mainstream labels are also fueling the trend. In a newspaper ad this month, New Balance announced that it had named Teddy Santis the head of a new American-made collection. In the ad, Mr. Santis—the designer behind New York label Aime Leon Dore—was flanked not by chisel-jawed millennial models, but by a phalanx of elegant elderly New Yorkers. Nearly all wore their own clothes, apart from the New Balances on their feet.

Clothing labels have certainly used seasoned models in their print ads before. Ralph Lauren’s campaigns have long featured leather-faced Marlboro Men-types. In 2015, the French fashion house Celine tapped the beloved writer Joan Didion—then 81—to model its designs. And elegant older models such as Maye Musk (72) and Carmen Dell’Orefice (89) have been prominent for some years now.

But in most of these precedents, the label in question carefully orchestrated the models’ outfits. What’s new is how younger generations are captivated by the way the elderly actually dress themselves. On the surface, Grandpa Style appreciation resembles normcore, the aughts trend of dressing in banal basics—such as gray sweats and nothing-to-see-here dress shirts—as a way of turning one’s back on the frenetic fashion market. Elderly style role models often hopped off the trend train years ago, so their fashion sense skews more elegantly traditional than normcore. These 70-somethings could still be wearing the same crisp pleated khakis or tweed blazers or floral sundresses they’ve had for decades and view them as “this old thing?” But to the youthful gawker who’s looking for alternatives to lurid logowear or slouch sweaters, such clothes are invigorating.

Some credit for elder appreciation is due to Ari Seth Cohen, a photographer who started “Advanced Style,” a more than decade-old blog showcasing the splashy style of elegant and eccentric 60-plus women. In their flouncy gowns and saucer-like hats, these style setters dress with maximum flair.

What makes the style of the elderly compelling is how they forge their own path. “They’re not worried about trends,” said Mordechai Rubinstein, a New York-based photographer who has for several years captured on his Instagram page the city’s well-dressed elders, who generally eschew logo-heavy luxury wear and attention-seeking sneakers. Mr. Rubinstein’s elderly urbanites really know how to use a well-placed accessory to make a humble outfit sing. In a shot from last November, an elderly gentleman wears workaday khakis and a traditional double-breasted blazer, but tucks his brown scarf idiosyncratically inside the coat—an exclamation point on his whole get-up. Another photo spotlights a passerby’s prismatic, peacock-feather-ish bucket hat.

“Chinatown Pretty” likewise captures the tantalizing flourishes of the stylish elderly. The book’s portraits capture standout floral cardigans, pastel-colored polos and weathered corduroy vests that would make vintage sellers salivate. The book’s subjects dress in a pleasing “patchwork” of layers, colors and eras, said Ms. Luu, who noted that many folks they shot were wearing items that they’d owned for decades—or, in some cases, brought over when they immigrated to America.

Some 20- and 30-something find that timeworn, and time-earned, fashion sense appealing. Though he’s part of the fashion world, Mr. Kivijarvi of @Gramparents said that contemporary fads rarely pull him in. After working a side job at a retirement home several years ago, he came to appreciate the winsome style of certain elderly people. “It looks so effortless and fresh because it’s such a different view of how we view our world of style and fashion,” said Mr. Kivijarvi, who often wears light jeans, New Balances and the occasional cheery cap, clothes that would get him featured on his own account—if only he weren’t so young.

Mr. Kivijarvi noted that fresh-faced followers often post photos of themselves in versions of the elders’ outfits he showcases, tagging his account. In 2019, a college-aged fashion influencer even sent him a YouTube video called “Copying Your Grandparents Outfits for a Week,” in which she dresses in outfits inspired by ones on @Gramparents. In the video—which has been viewed nearly 65,000 times—the young woman layers a denim jacket over a shin-length sundress, accessorized granny-style with a bucket hat and sandals over socks. In June, one of Mr. Mr. Kivijarvi’s new initiatives will inevitably boost this aping-the-eldery trend. In partnership with Another Aspect, a small Copenhagen-based brand, he’ll release a clothing collection inspired by outfits he’s highlighted on @Gramparents.

Meanwhile, the mainstream fashion industry still seems to take this mature demographic for granted. That may be misguided. In 2019, Internal Longevity Center (ILC), a British think tank that studies aging and its impact on society, released a study that found that people 50 and older are responsible for around 21% of spending on clothes and shoes in the U.K.. Nevertheless, said Sophia Dimitradis, a research fellow at the ILC who worked on the study, “the fashion industry ignores old people in marketing and also in terms of the type of products.” She says that the elderly consumers she’s interviewed feel that the industry’s offerings do not account for their changing body shapes and that ads, typically populated with the young, suggest that “stylish clothes are just for younger people.” Accounts like @Gramparents and books like “Chinatown Pretty” are busily disputing that.

Write to Jacob Gallagher at Jacob.Gallagher@wsj.com