Nearly another third of this age group have postmortems that reveal significant hallmarks of dementia - known as plaques and tangles in the brain - but seemed just fine while alive.

"How can people function at these higher levels? Science has not been able to answer that," says Loewenstein.

"And that is what we are trying to figure out."

Staying busy

Of the 100 people enrolled in Loewenstein's study so far, more than 40 live at East Ridge, a retirement village that resembles a typical suburban neighborhood in south Florida, with wild peacocks roaming beneath the palm trees, people driving around the manicured grounds on golf carts, and rows of single-story homes divided into multiple apartment units.

Such tranquility does not come cheap. Residents must pay $111,000 up front, then a monthly rent of $2,700 or more, depending on the size of their living space.

Soon after arriving seven years ago, Gwen North, a retired kindergarten teacher who appears decades younger than her age of 85, took on the responsibility of running the thrift store.

"I work probably six days a week," she says, happily.

At age 86, her husband Art is known as the go-to-guy around town - perpetually ready to chat, share information, or fix electronics that have broken.

Art and Gwen have already taken memory tests and are giving samples of their spinal fluid so that it can be studied for the earliest biological markers of aging. They have even arranged to donate their brains for further study after they die.

'Exercise in a pill'