“I’m not 20. I don’t want to be 20, but I’m really freaking cool. That’s what I think about when I’m posting a photo.”

Photographed with a hip thrust forward to show off her Margiela apron dress and modishly frayed jeans, Lyn Slater projects a kind of swagger pretty rare among her peers. A professor at the Graduate School of Social Service at Fordham University, with hyper-chic side gigs as a model and blogger, she is known to a wider public as an Instagram idol.

Sure, she’s 64 , a time when some women her age are feeling pressed to close up shop. But if you are Ms. Slater, that’s not going to happen.

On Accidental Icon, her influential Instagram account, she tends to vamp in an eye-catching mash-up of Comme des Garçons, Yohji Yamamoto and consignment store finds. Her following, hundreds of thousands strong, skews young, she said, and is responsive to her sass.

“I flaunt it,” she said. “I’m not 20. I don’t want to be 20, but I’m really freaking cool. That’s what I think about when I’m posting a photo.”