NEW YORK - At exercise class, 90-year-old Terry Drucker lifted her slim legs higher than her decades-younger classmates. Afterward, they all shared a champagne toast in honor of Drucker's recent birthday.

"I wouldn't be in this shape if it weren't for this class," Drucker said Tuesday at her post-exercise surprise party at Back in Shape studio off Manhattan's Fifth Avenue.

"Feel the difference between tension that makes you stronger - and just tension!" instructor Marjorie Jaffe urged her seven students, many of them urban professionals half Drucker's age.

Drucker first started exercising seriously in the 1970s after a back injury, taking a class Jaffe taught at the YWCA.

"She did wonders," Drucker said.

"I really believe that working out at 50 is an investment (for) when you're 90," said the retired Time Inc. researcher and reporter, who had contributed to a book series titled "This Fabulous Century."

Drucker's 59-year-old fitness expert is a published author - "The Muscle Memory Method" - who has taken her spa regimen on the road everywhere from Italy and the Caribbean to the Hamptons on Long Island. She bills the regimen as an exercise program that works every muscle while training the mind to operate the body correctly.

On a sunny morning, it's much livelier than that.

"Maurice Chevalier was the man I was crazy about as a teenager!" announces Drucker, flat on her back between some yoga-like stretches.

"Never mind about exhaling! Let's think about men!" Jaffe shoots back, sailing barefoot across the floor, her red toenails gleaming. "Let's go, Terry! Roll out to the side and let go. ... Let me hear your breathing..."

With a CD of classical music setting the mood, the students engaged in "girl talk" and giggles between moves. Grunts and groans rose from the green mats that line the wooden floor before a fireplace.

When the phone rang, a baby's voice was heard on the answering machine. "Oh, that's my granddaughter," Jaffe explained, before leading a weightlifting exercise to strengthen the backs of the arms.

Drucker is the oldest of Jaffe's students (three others attended class into their late 80s). She will now get her twice-weekly sessions free of charge; other students pay $20 per class.

Drucker said her determination to stay in shape comes after successful cancer surgery last year.

"It's helpful to your body and to your head to do this - it makes you feel good," she said.

Classmate Claire Meyer assured Drucker of her continued longevity, telling her with a toast, "To the next 10 years!"

And when she hits 100, Jaffe added, the group will send a car and driver to pick her up before the party.

Drucker, who has a teenage grandson, said the group is another positive factor about exercising at 90: "It's like having a second family. It's a group of people who care about you."

Said caterer Richard Hoyt, who set up the birthday cupcakes and tea sandwiches: "She's older - and she's very youthful. It's called joie de vivre!"