As dawn breaks over the world's largest metropolis, Keizo Miura, a sinewy centenarian, is already dressed in his charcoal gray tracksuit and pumped to sweat.

Before a hearty breakfast of seaweed and eggs, Miura races through his indoor exercises, wincing as his neck -- still tender from a collarbone injury -- momentarily reminds him that he was born in 1904. The man who has become a role model in graying Japan sucks it up, shaking off the pain the way he did last year when he skied down Europe's Mont Blanc at age 99. In a Tokyo minute, he is out the front door for his daily two-mile power walk.

"I still feel good," said Miura, who in 1981 became the oldest man to scale Mount Kilimanjaro, Africa's tallest peak, and is training for an expedition to the Italian Alps next year. "There's really nothing so amazing about me . . . but my son, now he is amazing."

That would be Yuichiro Miura, 72, who in May 2003 became the oldest man to reach the summit of Mount Everest after a two-month assault on the world's highest peak.

The Miuras are among the fast-growing ranks of super-seniors -- Japan's extraordinarily fit old folks. In a country where the average life span has extended to 81.9 years, Japan's elderly are not only the longest-lived but statistically the healthiest seniors in the world. The typical Japanese now enjoys at least 75 years of relative good health, according to the World Health Organization. That exceeds by nearly six years the average for Americans -- who rank 23rd -- and by three years the average for the French, whose seniors are warming the benches in seventh place.

Inside Yuichiro's bustling office in hip Harajuku, a Tokyo neighborhood packed with spiky-haired, nose-pierced teenagers, the robust and bright-eyed climber said he and his father are not alone in this nation's astoundingly healthy class of senior citizens.

"Older Japanese are remarkably healthy, doing things at their age that most youngsters couldn't do," said Yuichiro, who looks as if he could do arm curls with a neighborhood punk in each hand. "People over 65 here are climbing mountains, going to China to plant trees, traveling abroad to teach Japanese. It's about diet, it's about exercise. . . . It's about making the most out of a long life."

The extraordinary number of robust seniors means that many are able to embrace their twilight years with a gusto and sense of adventure once reserved for the prime of life.

This year, a group of 11 Japanese retirees -- with an average age of 63 and a team leader clocking in at 78 -- walked across China's Taklimakan Desert along the ancient Silk Road. For 73 days, they trekked about 750 miles in the steps of the 19th-century Swedish adventurer Sven Hedin, braving evening frosts with temperatures reaching 15 degrees below zero.

This month, Minoru Saito, 70, set sail in hopes of becoming the oldest man to circumnavigate the world alone without stopping at a single port. In May 2002, Tamae Watanabe became the oldest woman to reach the top of Mount Everest at age 63. As of December 2003, two Japanese in their seventies, 22 in their sixties and 44 in their fifties had climbed 8,000-meter-class (26,000-foot) Himalayan mountains, according to the Japan Himalayan Association.

A popular TV commercial features Minoru Nozoe, 68, a farmer, doing gymnastic twirls on the high bar. Almost half a million seniors gathered last month in Gunma prefecture for a sort of elderly Olympics where they competed in martial arts such as kendo as well as soccer, swimming and the marathon.

The Japan International Cooperation Agency, an organization similar to the Peace Corps, has seen a sharp increase in the number of seniors volunteering overseas in the past decade. Japanese retirees are now helping to develop new sewage systems in Bolivia and modern farming techniques in Cambodia.

With one in five citizens older than 65, spending by seniors -- who on average have far more savings than most of their peers in the developed world -- drives rising consumer demand as Japan emerges from a 13-year economic slump.

Without doubt, not all elderly Japanese are healthy, and their longevity is both a blessing and a curse. Along with the nation's low birthrate, covering medical and pension costs for seniors is widely viewed as the most significant long-term problem confronting Japan. By 2017, an estimated 27 percent of the population will be older than 65, rising to 35.7 percent by 2050, according to the National Institute of Population and Social Security Research.

But government projections indicate that relatively robust seniors may at least help blunt some of those costs. Despite the high cost of living, for instance, medical costs per person for Japanese older than 65 are still slightly less than those for their American counterparts -- about $6,500 per person in Japan compared with $7,055 in the United States, according to government statistics.

"Japanese seniors are not only living longer but their health is generally excellent, and as a group, they appear to be getting healthier," said Koichi Ando, assistant director of elderly affairs at the Health Ministry. "They are doing more and more exercise, while younger Japanese are spending more time sitting and scanning the Internet."

Studies indicate a multitude of reasons for the health of older people, with most citing a traditional diet heavy on fish and light on red meat, as well as the consumption of high-fiber rice. A national survey in 2000 showed that almost 63.6 percent of seniors don't overeat, 49.6 percent exercise regularly and 64.2 percent sleep well.

Older Japanese additionally have lived through the hardships of World War II and its aftermath -- and, in some cases, through the difficulties of World War I and the 1904-05 Japanese-Russian war. Those periods, geriatrics experts say, toughened older Japanese -- and they stayed tough even as Japan evolved into the world's second-richest nation after the United States.

In rural areas, the elderly tend fields and gardens for hours a day. Urban seniors, meanwhile, live active lives in cities such as Tokyo, where getting from place to place is often easier without a car. Despite Japan's high-tech society, most subway stations do not have escalators -- meaning substantial walking and stair-climbing, whether people like it or not.

"As opposed to America, seniors in Japan do not have to purposely go out and seek exercise -- everyday life makes them more slim and healthy," even while they maintain very high nutrition, said Makoto Suzuki, a professor of human welfare at Okinawa International University. "It's a winning combination."

That combination holds true even for the very old. In 2002, the United States, with a population of 283 million, had roughly 50,000 centenarians, but only about 13 percent of them were living independently. In contrast, in Japan, a nation of 128 million, there are 23,000 centenarians, with about 35 percent of them living independently, according to government statistics and research studies in both Japan and the United States.

A government-backed national health system also makes low-cost, high-quality medical care available to every citizen. But society is grappling with how to continue to pay the price as society ages.

Since 1973, citizens older than 70 have received subsidized medical care. But Japanese families, which traditionally care for their elderly at home, are finding their own expenses increasingly high as their relatives live longer.

Japan's pension system, meanwhile, has reached its breaking point -- hitting a record deficit of $7.7 billion last year. The shortfall forced the government into a highly unpopular overhaul of the system this year in which younger Japanese and their employers will be forced to gradually increase the percentage of their salaries that goes into the system, from 13.58 percent today to 18.30 percent by 2017. Some critics say even that overhaul may not be enough to avert a fiscal crisis.

"Japan's birthrate is very low -- and it begs the question of whether there will be enough actively working young people over time to cover the costs of the expanding aging population," said Yasuhiko Yamazaki, a pension expert at the Kanagawa University of Human Services. "The issue of how to support our long-lived elderly is going to remain Japan's most vexing problem."

The Health Ministry is working with the Miuras to develop a nationwide health program in which the elderly would receive subsidized gym memberships as well as training at senior centers. "We hope to make older Japanese stronger so they can live independently, caring for themselves in order to relieve the pressure on the nation," Ando said.

Keizo Miura, a longtime forest ranger, had not spent a single night in a hospital until this year. After climbing hundreds of mountains and skiing thousands of slopes, he slipped and fell in July while training. He cared for himself, living independently, even after his wife died in 1993 at age 80. Now, however, one of Miura's daughters has moved in to help him temporarily.

Even with his still-healing collarbone, Miura is strict about his exercise ritual -- now the subject of two books in Japan. He begins lightly, with "jaw stretches" -- opening and closing his mouth like a fish gasping for breath. "It's to keep my skin in good condition," said Miura, who uses a hearing aid. "And to have fewer wrinkles."

Neither Miura nor his son, Yuichiro, describes himself as a health freak. Rather, Yuichiro said, it is more about a desire to "live life with passion."

For father and son, that passion is mountain climbing and skiing, these days funded mostly by corporate grants and Japanese media outlets. While his father is planning on three weeks in Canada next month to prepare for Alpine skiing next year, Yuichiro -- who has already climbed the tallest peaks of the seven continents -- is busy opening a new low-oxygen training center in Tokyo. He plans to train there for his next attempt at Everest, set for 2008.

"I'm not thinking about my age, because this isn't about age," he said. "You know what it's about? It's about wanting to get to the top."

Special correspondent Akiko Yamamoto contributed to this report.

At 63, Tamae Watanabe became the oldest woman to climb to the top of Mount Everest. Keizo Miura, 100, does his exercise routine -- the subject of two books. He is training for a trek in the Italian Alps.Yuichiro Miura, 72, last year became the oldest man to conquer Everest. He wants to "live life with passion."