TOKYO -- With the longest life expectancy on Earth and one of the lowest birth rates, Japan's population is rapidly growing older. For a country with acute labor shortages already, this trend clearly threatens Japan's manufacturing might.

But such heavy concerns seem to have been forgotten for the moment amid the media hoopla surrounding the strangest manifestation of Japan's "aging boom": This country has gone wild over a pair of 99-year-old pop idols.

Even in Japan, where powerful national newspapers and TV networks can spread new fads the length of the land in the blink of an eye, the success of the twin sisters Kin-san and Gin-san, both widowed great-grandmothers, is considered amazing.

Kin and Gin (both names rhyme with "green") appear on some TV show or another virtually every day. Their new record is heard constantly on the radio and in sing-along karaoke bars.

The daily entertainment newspapers and weekly magazines, normally devoted to breathless reports on which cooing, miniskirted teenage singer is dating which hulking young sumo wrestler, are suddenly filled with articles on the aging idols and their favorite food (tuna sushi), drink (health tonics) and TV show (pro wrestling).

In a sense, the popularity of the centenarian celebrities is easy to explain. "The two of them," says Yukihiro Hoshino, the publicist for the twins' record company, "are just so cute." And this serious, hard-working country is crazy about cute.

But the popularity of Kin Narita and Gin Kaniye also reflects basic demographics. The lively, charming idols have emerged as the prime symbols of Japan's aging society.

People are living longer here; the female life expectancy is 82 years, and male 76. The younger generation is not producing enough babies to renew the population. Thus Japanese society is shifting rapidly upward on the age scale.

With 12 percent of its population over 65, Japan has edged past the United States in terms of proportion of the elderly in the overall population. U.N. projections show this island nation will pass Sweden within a decade to become the oldest population on Earth.

It was almost inevitable that somebody would emerge as the heroine of the super-elderly generation. The cheerful, talkative Kin-san and Gin-san are filling the role with gusto, dashing daily from this radio talk show to that TV drama to the filming of a new commercial.

Just as with America's Clara "Where's the beef?" Peller, Kin and Gin emerged as national stars on the basis of a single popular TV commercial.

The two widowed great-grandmothers were born in August 1892, when kimono-clad men with samurai topknots still walked the streets of their home town, Nagoya.

By the traditional Japanese reckoning, which counts an infant as 1 year old at birth, they are already 100 years old. Japan adopted the Western age-counting system after World War II, but since Kin and Gin were already almost 60 then, they didn't bother to switch.

The concept of 100-year-old twins caught the fancy of the advertising director at Duskin, a company that sells cleaning supplies. Duskin's phone number is 100-100, and the firm decided that a pair of adorable 100-year-olds could plant that number indelibly in the nation's mind.

Kin (the name means "Gold") and Gin ("Silver") made their national TV debut in a 15-second Duskin ad late last year, wearing traditional dark kimonos and kneeling on traditional Japanese floor pillows.

"I'm Kin. I'm 100," said Kin. "I'm Gin. I'm 100," said Gin. "And Duskin's number is 100-100," intoned the announcer. End of commercial.

In this homogenous, one-big-family society where everybody tends to think alike, it doesn't take much to start a new fad. And that single ad was enough.

The tiny twins -- each is 4 feet 2 and weighs about 70 pounds -- came across as lively and lovable. Quickly besieged by the media, they turned out to be alert, witty, loquacious and modest as well. Even today, with their stardom clearly established, both women manifest a sort of "pinch-me" attitude toward their media ascendancy.

When Kin, who has no teeth, choked on a piece of fish and was admitted to the hospital for observation, national networks and newspapers set up a 24-hour stakeout on the hospital lawn, with regular updates on Kin's condition.

But when she walked out of the hospital on her way home, Kin seemed astonished that anyone could be so interested in her. "You're all waiting for me?" she asked, and then she laughed at the very thought of it.

Last month the twins, surrounded by the now standard media horde, had to go to the tax office in Nagoya to report their earnings -- about $40,000 each -- for 1991. "This is great," said Gin. "I've always wondered what it's like to file a tax return."

The media here now routinely refer to the twins as the "100-year-old idols." This is itself something of a joke, because ai-doe-lu, or idol, is a term of art in Japan's entertainment industry. The word normally refers to the earnest, attractive high-school-age singers who are marketed to the teeny-bopper set.

In addition to the generational appeal and the twins' natural warmth, analysts of pop culture here cite at least two more reasons for the enormous popularity of the centenarian starlets.

"Japan is becoming an aged society, and people are worried about what's going to happen to them," says Hiroshi Shibata, of Tokyo's Metropolitan Institute of Gerontology. "But Kin and Gin are so full of life; they are not senile at all; they've lived to 100 in great shape. That's an inspiration to everyone else."

Moreover, Kin and Gin are thoroughly Japanese.

The pop world here is full of stars and groups who use English stage names (Anna, Bo Gumbo, the Kids, Sky Walker, Ladies Room), fill their songs with English words and basically do everything they can to look as if they just got off a plane from Los Angeles, the ultimate cool place for many Japanese young people.

Kin and Gin represent the reaction to this westward lunge. They speak no English and wear no Western clothes; they are always garbed in kimono, sandals and traditional socks. Gin does like to eat hamburgers, but that is so common here it's not considered foreign anymore.

These days, the twins are busily promoting their new record, "Kin-chan to Gin-chan" ("Little Miss Gold and Little Miss Silver"), their new photo book, their latest TV drama and whatever other projects their managers think up. In America this would create a problem of overexposure. But in Japan's pop culture, where more is more, Kin and Gin seem immune.

"Oh, Kin and Gin, I love them," says Yuko Ariyoshi, a suburban housewife some seven decades younger than the twin stars. "I read the TV guide, and whenever they're going to be on, I try to watch."

Asked to explain her sudden fascination with two women who could be her great-grandmothers, Ariyoshi responds without hesitating, giving the answer in a single word: "Kawaiiiiiii."

In English, that would be "cuuuuuuute."