"How old are you, Sister?"

The question brings a sly smile to the face of Sister Agnes Barbara Hettel. "I'm 92," she replies. "I'm one of the young ones."

That's an inside joke. And David Snowdon gets it. Probably only God knows more elderly nuns.

Spring and the promise of new life are in the air as Snowdon -- high-profile professor of neurology at the University of Kentucky College of Medicine -- makes his way through the halls of Villa Assumpta, a converted mansion just north of Baltimore that serves as local headquarters for the School Sisters of Notre Dame.

About 80 retired nuns live here. Most are former teachers and administrators. In younger days, they lit fires under students in Catholic schools around Baltimore and shaped careers at the College of Notre Dame.

Some keep busy working at the reception desk or maintaining archives. Some, like Sister Agnes Barbara, are full-time occupants of the third floor, a licensed medical unit.

That's where Snowdon is now. At 54, he still has an adolescent bounce to his step as he walks the corridor. He could be a politician courting shut-ins, a battlefield general bucking up troops.

"Hi, Sister, how are you? ... Thanks for all your help. ... Keep me in your prayers, please."

Twenty years ago Snowdon began groundbreaking research that now encompasses nearly 700 School Sisters across the country, including those he's chatting up today. As participants in his "Nun Study," these sisters undergo annual physical and mental tests that gauge the effects of aging. Snowdon will track them until the day they die -- and beyond.

Their hope is to unravel some mysteries of Alzheimer's disease, the creeping dementia that affects 4.5 million Americans and, at a cost of $100 billion annually, ranks among the country's most expensive health problems.

The School Sisters in Snowdon's study left the classroom behind long ago. Now they're teaching a larger audience what it means to age well and with dignity.

That's a lesson worth imparting. America, like the School Sisters, is going gray. Millions of baby boomers are completing their long, strange trip from rock 'n' roller to Social Security recipient. And potential Alzheimer's patient.

Over time, the fortunes of Snowdon and his School Sisters have become intertwined. Like strands of a church bell rope. Like two hands clasped in prayer.

The Nun Study has been the subject of a Time magazine cover story, a Nightline broadcast, radio shows, innumerable articles and one book, Aging With Grace, written by Snowdon.

It has been credited with two landmark findings: establishing a link between vascular episodes, such as stroke, and the onset of Alzheimer's, and confirming a belief that intellectual activity helps ward off the disease.

The study not only shed new light on Alzheimer's, but also bent the usual rules of engagement that separate researcher from subject. Snowdon and the sisters turned into friends. Their story is as much about the simple humanity of their relationship as it is the complexities of science.

For the School Sisters this is just another mile marker on a long road of service. Not for Snowdon.

"I don't know if there's going to be anything quite like it done again," he has said.

Continuing his rounds, Snowdon taps on Sister Helen Fellenz's open door. She is 94 and for many years taught music at the order's schools in Japan.