If you thought retirement planning is hard, you ain't seen nothing yet. Wait until you get a glimpse of "centenarian planning." Oooompf! Better pull up a rocking chair.

The good news is that centenarian planning isn't for everyone. In fact, the odds still are pretty good it's not for you. The bad news is that if you are one of the vast majority of adults for whom centenarian planning won't matter, the reason is . . . do you need this spelled out? You won't live long enough.

On the other hand, population and medical experts report that, thanks largely to remarkable progress in medicine, health and education, one undeniable trend in aging today is that more of us than ever are living past the brittle old age of 100. And the number of joint-aching centenarians (100 years and older) is expected to continue increasing dramatically over the next 50 years, so much so that the 1998 United Nations population estimates and projections for the first time added numbers worldwide for what it calls "the oldest-old" -- people 80 years and older.

Though still only 1.1 percent of the world's population, oldest-old totals are projected to increase sixfold by 2050 -- from 66 million last year to 370 million. Folks actually reaching age 100 and beyond are expected to jump from an estimated 135,000 in 1998 to 2.2 million in 2050 -- making them the fastest-growing class of oldest-old timers.

The number of U.S. centenarians today is estimated at about 70,000, and is expected to increase more rapidly than worldwide rates over the next 50 years. All of which brings to mind an image of a 100-year-old Willard Scott on some future "Today" show sending out birthday greetings on any given morning to dozens and dozens of centenarians.

"It has been predicted that 3 million baby boomers will make it to 100," says neuropsychologist Margery H. Silver, associate director of the New England Centenarian Study and coauthor with Thomas Perls of "Living to 100: Lessons in Living to Your Maximum Potential at Any Age" (1999, Basic Books, $25). "It has also been predicted that every female child born today has a 50-50 chance of living to be 100."

A clinical instructor in psychology at the Harvard Medical School, Silver says one conclusion to emerge from the many myth-defying findings of this ongoing medical and demographic study of centenarians in the Boston area is that if you are going to live to be 100, you have to plan to live a healthy and forward-looking lifestyle. Not only will that help you reach a ripe old age, it will make living la vida del centenario more worthwhile if you do.

But how at midlife do you even begin to think about the implications of possibly having another 50 years ahead of you? "People really need to be planning not just in terms of a financial portfolio, they need to think about a `life portfolio,' " says Silver. "That means thinking about your time and personal resources, and how you want to allocate and spend them with this tremendously expanded life span."

In their 1998 book, "The Longevity Strategy: How to Live to 100" (Wiley & Sons, $16), David Mahoney and Richard Restak promise to provide "everything you will need to know to increase your chances of becoming a vigorous centenarian -- living to be 100 years old and liking it."

A clinical professor of neurology at George Washington University, Restak says a number of factors within our control can determine our chances of reaching the personal century mark -- as well as whether we stride past 100 chipper and alert or barely crawl to the finish line. "We have a lot of insight into the not-to-do's and the to-do's," says the neuropsychiatrist in private practice in the District. "In terms of a formula, exercise and diet are important, mental attitude is important . . . ."

Indeed, many of the factors are the tried-and-true elements of a healthy lifestyle at any age. But not all. Some recommendations on to-do lists for would-be centenarians defy conventional perceptions of old age.

For instance, one of Silver's findings in the New England study is that a quarter of its 100 or so centenarians were "cognitively intact," and another substantial number "were thinking quite well," she says. "That went against the common thinking that by the time you get to be 100 everyone will be demented."

The lesson from those who were sharp as a tack at 100? "Keep mentally active," says Silver, recounting how one of her centenarians, at 104, learned an entirely new branch of mathematics and started writing articles about it. "Exercising your brain is as important as exercising your body. The key is new learning. It really appears that when you learn new things, you brain grows new connections between the neurons, and that gives you a kind of reserve that helps you not succumb so easily to some of the negative changes that might take place due to aging."

As for physical well-being, Silver acknowledges that some people have the genetic edge in reaching 100 -- but lifestyle choices count for a lot. "Most of us have the genetic factor to live well into our eighties," says Silver, "so we really need to pay attention to taking good care of our health."

Making preventive health choices in our lives early on -- such as not smoking, avoiding saturated fats, drinking only moderately, keeping trim, avoiding too much sun, exercising, and taking the right vitamins -- can add 10 or more quality years to anyone's life, explains Silver. Of course, not doing those things can subtract years.

To help people plan for extended and healthier lives, New England Centenarian Study director Thomas Perls designed "The Living to 100 Life Expectancy Calculator" -- www.beeson.org/Livingto100/default.htm -- an online quiz that calculates how old you are likely to live based on what has been learned in centenarian and longevity research.

"All the questions are weighted differently. And it has some surprises, like who would guess daily flossing can make a difference," says Silver, referring to recent scientific evidence that chronic gum disease releases toxic substances and bacteria into the blood stream that can cause plaque formation in arteries, leading to heart disease and increased risk of stroke and accelerated aging.

But living to 100 also requires career and financial planning, according to Mahoney, who is chairman of The Charles A. Dana Foundation, a brain-research think tank, and Restak, his coauthor. "By your forties, you've got to start thinking about a second and third career for later in life and start building certain skills," says Restak, adding that these centenarian "avocations" need to be playful -- such as painting, philanthropy, writing, political activism, or antiquing. "It should be linked into your spirit, if you will. How many years can you do the same old thing? It should be something that makes you happy, that you can be rewarded for doing -- both financially or otherwise -- by the time you're 95."

The wrong route, he warns, is taking early-retirement without re-dedicating yourself to a new passion: "The serious avocation stimulates the brain."

Mahoney and Restak also recommend planning for financial independence that can last an additional 20 or 30 years beyond what most people think of as a lifetime. They warn against relying on social safety nets such as Social Security payments, and instead start reducing debts early in life and invest heavily in IRAs and retirement plans that focus on long-term capital growth.

But Silver says many centenarians invested in something less tangible than the stock market, but that pays off in emotional support: family and social relationships. "Contrary to typical thinking that older people are going to be very old and alone, the centenarians have strong families and good relationships with many friends," she reports. "And even those with no family create new family situations" that enrich their lives.

Centenarians also typically have a great sense of humor, adds Silver, who recalls one 100-plus woman who, when asked about the touchy subject of donating her brain to science, replied, "But I'm still using it!" Research shows that emotional levity, finding the humor in things, has a positive effect on health. "They actually found that laughter is like internal jogging," she says, "with effects on the cardiovascular and immune system. So it certainly can contribute to people staying healthier and living longer."

After 14 years of studying centenarians, Lynn Peters Adler has come to recognize what she calls the "Centenarian Spirit," which includes factors she believes are important to the longevity of these remarkable elders.

"These are things that everyone can start developing," says the founder of the National Centenarian Awareness Project, and author of the 1995 book "Centenarians: The Bonus Years" (1995, Health, $25), mentioning a combination of a positive yet realistic attitude, a love of life, a continued involvement with others, and a strong spiritual or religious belief.

An extended life span, says Adler, also requires personal courage and self-determination. "That includes the willingness to use medical interventions when necessary," she says. "People have had mastectomies in their eighties or nineties and still live to 100. But they never think they won't get through it."

The most important characteristic? "A remarkable ability to renegotiate life at every turn, to accept the inevitable losses and changes that come with aging and not let it stop them," says Adler, mentioning devastations of living a long life such as losing one's spouse, illness, even losing one's children. "Centenarians are not quitters. In their approach to life, when things go wrong, they roll with the punches and they do go on.

"We need to learn to accept those changes, and make plans for how we're going to handle them when they happen to us -- so we can keep on going. Because the only secret to living to 100 is surviving your seventies, eighties and nineties. And enjoying it -- because what's the point of living to 100 if your aren't going to enjoy it?"