Sophy Curson, the stalwart Rittenhouse boutique, celebrates its 90th birthday | Elizabeth Wellington

Susan Schwartz is dressed for a party.

The longtime owner of the Sophy Curson boutique on 19th Street is luncheon-cute in a hot pink Tom and Linda Platt shantung tunic accessorized with a jeweled, vintage Christian Lacroix belt. Her black trousers fit loosely. And her salt-and-pepper hair is reminiscent of Diahann Carroll, an elegant pouf with sharp, tapered sides.

“I still like to dress up,” says Schwartz, who, at 82, is the grand dame of all that is fashionable in Rittenhouse Square.

Schwartz, who, these days, considers herself semiretired, is in town to chat about Sophy Curson’s 90 years in business. And the store’s windows, designed by Dana Morelli, feature nine mannequins dressed in Tom & Linda Platt gowns to represent the popular trends of the store’s nine decades in business, starting with the 1920s flapper through to 1990s feathers and leather. One Linda Platt gown was made in honor of 9/11, featuring an American flag fashioned from Swarovski crystals on its bodice.

Sophy Curson will celebrate its 90th birthday with special trunk shows, including next weekend’s Catherine Regehr presentation.

It was Schwartz’s Aunt Sophy who opened the first store (next door to the current store) a few months before the start of the Great Depression. In its early days, Sophy Curson was a destination for petite women — Curson was under 5 feet tall — and, for decades, its trademarked slogan was, “Junior is a size, not an age,” but Curson eventually expanded to include sizes 2 to 20. Members of the wealthy du Pont family in Wilmington were Curson’s earliest customers and helped build the store’s cachet with the well-to-do.

Curson never married, but she had two sisters: Rosemond Price and Pearl “Mrs. G.” Goldner, who helped run the store for eight decades (as well as a store in Fort Lauderdale, Fla., which closed in 2012), offering of-the-moment pieces during the Great Depression, World War II, the Vietnam War, the Age of Aquarius, the Reagan years and post-9/11. Curson died in 2004 at 101 while her sisters Price and Goldner died in 2009, at 99 and 2011 at 102, respectively.

Curson and her two sisters worked part-time until their deaths, but it was Goldner’s daughter, Schwartz, who began doing the bulk of the buying in the mid-1970s, when she returned home to Philly after a stint as a sportswear editor at Harper’s Bazaar in New York. Schwartz says her favorite fashion eras were 1960s mod, 1970s knits, and 1980s Dynasty-inspired glam. In those days, she said, people really knew how to dress up.

In 1988 Schwartz’s son, David, joined the store, just before pantsuits got big. “We had them, but our customers just didn’t buy them up until then,” David Schwartz said. And the mother-son team started their three-decade run of dressing A-listers for the city’s swankest affairs, from the Friends of Rittenhouse Square’s Ball on the Square, to the Academy of Music’s annual Concert & Ball, coming up Jan. 25.

The Tuesday before, in fact, is the store’s “crunch day” said Schwartz, pointing out the sparkling showpieces from some of his favorites: Krizia, Catherine Regehr and Tom & Linda Platt’s evening wear collections.

I asked him about the well-known book he keeps with the names of his customers and the dresses they purchased for the ball — he can’t afford to duplicate dresses from his store on the Academy Ball dance floor, he said. “I’ve been known to hide dresses in my office,” Schwartz laughed.

These days the quaint store is packed with so many pieces that a shopper really needs a sales guide to work her way through the glitz. Schwartz holds up a short, black metallic dress from a new line called Grayse. Really cute. But where would you wear it in Philadelphia? I ask.

“Vernick at the Four Seasons, of course," Schwartz said, without missing a beat.

It is these understated, respectable, and classy labels — Italian Mariuccia Mandelli or Krizia — that aren’t particularly well-known but that keep the customers coming back. For 90 years.

Sophy Curson is located at 122 S. 19th St., 215-567-4662.