At an age when many physicians are thinking about retiring, Dr. Linda Bach is just starting her career. Last year, at 52, she was the oldest resident at Jackson Memorial Hospital in Miami. She is now in private practice.
She's keeping a promise she made to herself long ago. "My father died of a heart condition when I was 10 years old," she says. "It was traumatic to lose a parent at a young age. I decided right then that I wanted to be a doctor to save other little girls from losing their fathers."
But she was rejected from medical school and had to put her dream on hold for marriage, motherhood and a teaching career.
At 45 she decided to try again, and succeeded. She graduated from the University Of Miami School Of Medicine.
No one was prouder than her husband Bill. "The joy that's in her life is incredible to see," he says. "She gets so excited. She loves it; she's fulfilling a dream."
As the population ages, second careers are becoming more common, but few are as intense as Bach's.
Some days she struggles through a grueling 28-hour shift in the intensive care unit with doctors half her age. "We all get tired," says Bach. "There's no doubt about that. But I don't think I get any more tired. When the desire to do it is so strong, I think that overcomes the tiredness."
If anything, Bach's determination is an inspiration to her younger colleagues. They know that if they tell Bach they're tired, she'll tell them to "suck it up."
That Bach keeps up with her twenty-something colleagues is no surprise to Dr. Marilyn Albert of Massachusetts General Hospital. Her specialty is the aging brain.
"The more your brain is used, the more likely it is to be in its best condition," says Dr. Albert. "We believe the adage that says 'use it or lose it.'"
"I think we have always seen people who did very well, but what the research is telling us is that they don't have to be exceptions," she says. "This is possible for lots of people."
In a telephone interview with CBSNews.com, University of Illinois Professor William Greenough said he also believes in the "use it or lose it" adage, and he has done extensive research with rats to back it up.
"The brain is capable of forming new circuits, new pathways of neurons, throughout the life span of the rat," Dr. Greenough explains. "Moreover, nerve cells don't work in isolation. They are dependent on a network - consisting of a blood supply and glial cells - which allow the nerve cells to operate."
His research has shown that adult rats kept active in their environment were able to form new pathways of neurons in the brain, and just as importantly, displayed a more robust network to support these new connections.
"The 'use it or lose it' principle applies to the adult brain in two ways: first, its ability to keep previously acqured knowledge; and second, its ability to actively learn and retain new information," Dr. Greenough says.
His research suggests that exercising the human mind into old age keeps it strong and capable of continued learning, just as in rats. And he points to some related research by others with powerful implications: the onset of Alzheimer's symptoms is much slower in people who have attained a higher level of education.
Think of education as an agent for graceful aging.
She graduated from medical school in 1928, and shortly after helped develop the whooping cough vaccine.
She's not shy about sharing her views, and plenty of parents are eager to listen to her 70 years' worth of accumulated medical wisdom.
Dr. Leila Denmark, who at the age of 103 still practices medicine in rural Alpharetta, Georgia, is a case in point.
Dr. Leila Denmark with a patient.
Dr. Denmark credits her longevity to a love of work and healthy habits like taking regular walks.
How much longer does she hope to practice medicine? "I'll quit someday when I feel like I can't help these children," she says.
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