Blame it on boomers – people born between 1946 and 1965. The baby boom generation now makes up more than a quarter of the US population population -some 77.5 million people, with more than 160,000 in San Diego. One-third are over age 50.

Every seven seconds, another boomer joins that group. In just seven years, the first boomers will hit official retirement age. By 2030, boomers 65 and older will represent one in every five people.

For them, 65 will be the new 45. Or so they hope, and so many claim.

From self-described anti-aging institutes to miracle elixirs to how-to manuals for living a century or more, boomers (and just about everyone else) want to live longer than those in previous generations.

These days, the average American has a life expectancy of 76.9 years — a little more for females, a little less for males.

Most people, of course, want to live much longer than the average. But what are the odds of living to the century mark and beyond? And how much beyond is possible?

The good news is that most scientists think human life expectancies can be substantially stretched. The not-so-good news, some experts say, is that the estimated maximum human life span — about 125 years — seems fairly fixed and that most of us will never get close.

“Longevity is really a modern phenomenon,” said S. Jay Olshansky, a demographer and biologist at the University of Illinois at Chicago. “The vast majority of humans ever born died before the age of 10, usually from infectious diseases.

“We’ve done fabulous things to boost the survival rates of the young — improved sanitation, new medicines — but now it’s a whole different ballgame. It’s not so easy to add 70 years of life to somebody who’s already 70 years old.”

No doubt. But a number of scientists and doctors think it’s too early to start talking about a “finished” line. They assert, in principle, that there is no maximum human life span.

Aubrey de Grey, a biogerontologist at the University of Cambridge in England, says that under the right circumstances, humans born in the 22nd century (just 96 years away) could live up to 5,000 years.

De Grey, who advocates using technology to develop a “true cure for aging,” is indisputably at the optimistic extreme. But plenty of others see longer lives ahead.

“I think people will someday live substantially longer than today,” said Steven Austad, a biologist at the University of Texas Health Sciences Center in San Antonio. “(Living) into your 100s will be fairly routine, up to 150 for the outlier (a longer-lived person who is the exception to the rule). I think this because we have been so successful at figuring out how to make animals live longer.

“The arguments (against appreciably longer life spans),” he added, “are based so far as I can tell on ignoring a huge pile of research done over the past 15 years and the mystical belief that longevity, unlike every other human trait we know of, is impossible to change.”

Wear and tear

In biological terms, aging is usually defined as the accumulation of random damage to the building blocks of life, most notably DNA. The damage starts in early childhood and accelerates after age 30 or 40.

Over time, the accrued, unfixed damage impairs bodily functions. Cells, tissues, organs and systems work less well.

The immune system of a typical 65-year-old, for example, is only one-tenth as effective as that of a teenager. With less protection comes greater vulnerability to disease.

“Aging, in our view, makes us ever more susceptible to such ills as heart disease, Alzheimer’s disease, stroke and cancer,” Olshansky wrote, along with Leonard Hayflick, a gerontologist at the University of California San Francisco, and Bruce A. Carnes, a professor of geriatric medicine at the University of Oklahoma Health Sciences Center, in an online essay published this year by Scientific American magazine.

But conditions like heart disease are age-related, not the actual equivalent of aging, the scientists note. And aging is not the same as longevity.

Even if modern medicine could eradicate all the leading causes of death among the elderly, says Hayflick, an early pioneer in gerontology research, human life expectancy would increase no more than 15 years.

People would still age, he said. Other afflictions would rise up to exact their deadly toll. The maximum human life span would remain unchanged.

Age-old questions

If scientists want to boost that maximum life expectancy of about 125 years, most experts say, they’ve first got to solve the questions of how we age and why.

All organisms age, but the process, called senescence, is variable and, in some species such as the giant tortoise and rougheye rockfish, it’s virtually negligible.

The tortoise is known to live for 150 years or more; the rockfish more than two centuries. Both exhibit almost no signs of aging.

Variable senescence among species suggests to researchers that there are biological mechanisms, as yet undiscovered or understood, that might be altered, replaced or removed to effectively slow or even reverse aging in humans.

De Grey at the University of Cambridge says biotechnology is the answer.

He thinks current and foreseeable medical technologies, from drugs that repair or prevent cellular damage to organ regeneration and replacement, may soon be able to reverse the effects of aging. He predicts that researchers will actually do so in mice during this decade.

“Intervention to remove the accumulating damage . . . has the potential to postpone aging indefinitely,” he said.

A big key will be genes, which researchers say dictate and exert influence over roughly 30 percent of the aging process.

“As we begin to learn more about genetics, we see that there perhaps are certain genes that enable people to cope better with stress, react better to hormones and possibly regulate the rate of aging,” said Dr. Robert Butler, president of the International Longevity Center-USA, a New York City-based think tank.

Scientists are pushing hard to find such genes.

In 2001, Harvard University physicians and molecular biologists conducted tests on people who were all at least 90 years old and found they shared one or two genes on a specific chromosome. The exact function of these genes, however, has not been determined.

More recently, Olshansky and colleagues have launched a global project to identify so-called longevity genes by sampling DNA from exceptionally elderly people in places where very long life spans are common, such as Okinawa, Japan; the Vilcabamba valley in Ecuador; and the Hunza region of Pakistan.

Such efforts, though, won’t mean more birthdays for everyone anytime soon.

Most researchers are skeptical that there will ever be a one- stop genetic remedy. They note that aging involves lots of other factors, not to mention the considerable ethical and social issues attached to significantly modifying the human genome.

“Is the purpose of medicine and biotechnology, in principle, to let us live endless, painless lives of perfect bliss?,” the President’s Council on Bioethics asked in a report last year. “Or is their purpose rather to let us live out the humanly full span of life within the edifying limits and constraints of humanity’s grasp and power?”

The council expressed concern that a world full of centenarians and a diminished sense of mortality might result in problems no one can imagine or resolve.

Others suggest that such worries miss a more pertinent point.

Tom Perls, a geriatrician who runs the New England Centenarian Study, says most people are already genetically well-equipped to live reasonably healthy lives well into their 80s. The only requirement: They take good care of themselves.

Dr. Dilip V. Jeste, director of the Stein Institute for Research on Aging at UCSD, agrees.

“The obstacles over which we have control (of aging) are primarily environmental and behavioral,” Jeste said. “These include smoking, use of drugs of abuse as well as excessive alcohol, sedentary habits, poor nutrition, etc.

“The prevention of hypertension, diabetes and obesity may help increase life span significantly,” he said. “Resilience, optimism, adaptation to changing circumstances and optimal coping style are also important.”

Jeste said the focus of aging science should be less about extending life spans and more about making aging a “successful process associated not only with longevity, but also with a high level of activity of brain and mind.”

The record for the longest documented life is held by Jean Calment, who died in France in 1997. She was 122. The last years, however, were not kind. She was blind, deaf, incontinent and unable to care for herself.

Quantity wasn’t quality, and Calment most likely longed for the youth of her 90s.