Mary Harada’s father lived to 102, healthy and sharp to the end. She wouldn’t mind living that long, if she could stay as mentally and physically fit as he was. “He died sitting in his chair,’’ says Harada, 80, a retired history professor who lives in West Newbury, Mass. “He was in excellent shape until his heart stopped.’’

She may, in fact, have a good chance of getting there. Longevity experts believe that extreme old age — 100 or older — runs in families, and often is strikingly apparent in families where there are several siblings or other close relatives who have reached that milestone. (Harada’s great-aunt — her father’s aunt — also lived an extremely long life, to 104.)

Moreover, researchers are finding that many of those who live to extreme old age remain in remarkably good condition, delaying the onset of such chronic and debilitating age-related illnesses as cancer, heart disease and diabetes until close to the end of their lives, and a certain percentage don’t get them at all.

“It’s one thing to live to be 100 and quite another to live to be 100 and be in good shape,’’ says Winifred K. Rossi, deputy director of the Division of Geriatrics and Clinical Gerontology at the National Institute on Aging. The institute is sponsoring an ongoing study of more than 500 families with long-lived members that involves nearly 5,000 individuals. “Something is going on that has protected them from the bad stuff that causes problems for other people earlier in life.’’

( Martin Tognola for The Washington Post)

Experts attribute healthy longevity to a combination of good genes and good behaviors. Good behaviors play a greater role than genes in getting you to your mid-to-late 80s — don’t smoke or drink alcohol, exercise regularly and eat healthfully — while getting beyond 90, and to 100 or even older, probably depends more heavily on genes, they say. Families with a cluster of members with exceptional longevity don’t occur by chance, they say, but probably from familial factors they all share.

Growing numbers

Centenarians have become a fast-growing group in this country. In 1980, there were 32,194 Americans age 100 or older. By 2010, the number had grown to 53,364, or 1.73 centenarians per 10,000 people, according to the Census Bureau. This represents a 65.8 percent increase during that period, compared with a 36.3 percent rise in the general population.

Moreover, the number of Americans 90 and older nearly tripled during the past three decades, reaching 1.9 million in 2010, and is expected to more than quadruple between 2010 and 2050, according to the bureau. Globally, the number of centenarians is expected to increase tenfold during that time, according to the aging institute.

This is probably due to numerous factors, among them improved health care, dietary changes and reduced rates of smoking.

“When I started practicing, it was rare to see someone of 100, but now it’s not that strange at all,’’ says Anne B. Newman, director of the Center for Healthy Aging at the University of Pittsburgh. “More people have had the opportunity to get there,’’ largely because of advances in public health and medicine.

But as the numbers of very old have increased and the examination of human genetics has become more sophisticated, researchers have been trying to discover the genetic and biological factors that contribute to a life span of 100 or older and why some centenarians stay healthy for so long. Not surprisingly, what they are finding is complicated and far from a one-size-fits-all answer.

“Aging is not simple,’’ says Thomas Perls, a professor of medicine at Boston University and director of the New England Centenarian Study at Boston Medical Center. “There are many different biological mechanisms involved in aging, so it makes sense that there are different genes involved. We are still in the infancy of figuring this out.’’

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Nir Barzilai, director of the Institute for Aging Research at the Albert Einstein College of Medicine in New York, has been conducting several studies that focus on inherited genetic and biological influences that promote longevity.

In 2003, for example, his team discovered that centenarians, especially women, and their offspring have significantly higher HDL, or good cholesterol, which protects against heart disease, hypertension and metabolic syndrome, a series of risk factors that raise the chances of heart disease, diabetes and stroke.

The results, which found HDL levels of 60 and higher within this group — anything lower than 50 raises the risk of heart disease — suggest a heritable trait “that promotes healthy aging,’’ he says. This isn’t surprising, considering that women outlive men overall and — in 2010 — nearly 83 percent of centenarians were female, according to the Census Bureau.

Unusual chemistry

The Einstein researchers also have found that centenarians and their offspring often make unusually large amounts of a peptide (a short chain of amino acids) called humanin, which declines with age in most people and whose loss contributes to the development of Type 2 diabetes and Alzheimer’s disease. This may help explain why those who produce higher levels of humanin enjoy greater protection against those diseases and experience exceptionally long lives. For these individuals, humanin diminishes as they age, too, but the levels are much higher to start with than those of average people.

Barzilai believes the propensity for high levels of both HDL and humanin is heritable: “Offspring of centenarians have higher levels of humanin than their parents. Same with HDL. It declines with age, so it’s more apparent in the offspring.’’

Perls and his colleagues, in a study released almost four years ago, concluded there is no single common gene variant responsible for exceptional longevity. Rather, after examining about 280 gene variations, they discovered a series of gene combinations — nearly two dozen, in fact — that they believe contribute to long lives, “meaning there are different ways to get to these old ages,’’ Perls says. “It’s like playing the lottery. If you get all seven numbers, you’ll hit the jackpot.’’

These genetic groupings also seem to be involved in protecting against developing age-related diseases, since the scientists did not find an absence of disease-causing genes in their study group. “They have just as many as everybody else, which was a big surprise to us,’’ Perls says.

Also, the researchers found that the children of these healthy centenarians stay healthy longer than their same-age counterparts. The offspring of centenarians show 60 percent less heart disease, stroke, diabetes and hypertension, and 80 percent fewer overall deaths when they are in their early 70s, than those who were born at the same time but who do not have longevity in their families.

“They remain incredibly healthy into their 70s and 80s, and their mortality rate is very low, compared to others born at the same time,’’ Perls says.

Perls has studied 2,300 centenarians since 1995, including “super-centenarians’’ of 110 or older, and their offspring. He says about 45 percent of those who reach 100 manage to delay chronic age-related diseases until after they turn 80, and about 15 percent never get them at all.

Furthermore, he found that “semi-super-centenarians’’ — that is, those who are 105 to 109 — and super-centenarians don’t develop those diseases until roughly the final 5 percent of their very long lives. “They are dealing with diseases much better than the average person,’’ he says, who is more likely to develop these diseases in the 60s and 70s.

Many eventually die from the same diseases as non-centenarians, “but they do it 30 years later,’’ Barzilai says.

‘An additional 10 years’

Perls says that if you want to know whether you will live to 100, “you don’t have to do all this complicated genetic testing. Just look at your family and your health-related behaviors.’’ If you engage in healthful practices, you could reach your late 80s. “If you have the genes for longevity and you fight them [with risky behaviors], you will chop time off,’’ he says. “But if there is longevity in your family and you don’t do those things, you might get an additional 10 years past 90.’’

Newman agrees. “Don’t underestimate how powerful lifestyle is in longevity,’’ she says. “Even if longevity runs in your family, your life expectancy still will be more influenced by how you take care of yourself. If you have a centenarian parent, don’t count on living to 100 if you smoke, drink, eat a high-fat diet, and are sedentary and sleep-deprived.’’

Mary Harada thinks less about her genes and more about the unexpected event — breaking a bone, for example — that could make her a burden to her adult children.

“I don’t spend much time thinking about how long I’m going to live,’’ she says. “Whatever happens, happens. I spend more time thinking about how long I’m going to stay in my current house.’’

She has no age-related diseases and always has taken good care of herself. She has been a runner for 47 years, and she lifts weights. She shuns smoking and avoids most processed foods. She lives alone — her husband died last year — and she does most of the maintenance in and around her four-bedroom house, including leaf removal, routine yard work and spending two hours every 10 days in spring and summer mowing a very hilly lawn.

“I’ve lived here for 40 years, and I like living in this house and in this town,’’ she says. “If I could be like my father, and not break anything, I would stay here another five to 10 years. That would be wonderful.’’

Cimons — whose mother lived to be 100 — writes frequently about health issues for The Post.