America may experience a "centenarian boom," involving a surprisingly rapid growth in the number of people who live a century or longer, two reports conclude.

One, prepared by the statistical bureau of the Metropolitan Life Insurance Company, found that the chances of living to age 100 have increased by 40-fold since 1900. The odds doubled during the past 10 years alone.The other, prepared by the U.S. Census Bureau, predicts that America's current centenarian population of about 26,000 easily could quadruple by the year 2000, and be 40 times its present size by 2050.

"The centenarian population is likely to increase dramatically in the coming decades," said Dr. Gregory Spencer, a demographer with the Census Bureau. "The implications of this growth may be dramatic for pension plans, social welfare programs, life insurance or annuity plans, and the like."

Health and population authorities express similar concerns about the growth in the number of Americans now surviving to 85 or more - a group that represents the fastest growing segment of the population. Many extremely old people are chronically ill or disabled, requiring intensive and extremely costly levels of health care.

Spencer predicts that America probably will have 54,000 centenarians by 1990;108,000 by 2000; 441,000 by 2025; 1.03 million by 2050; and 1.9 million by 2080.In contrast, the best estimates that there were fewer than 4,000 centenarians in1960.

He emphasized that the estimates are not based on any unusually optimistic assumptions about future extensions in longevity of the elderly. Rather, they assume only modest gains in longevity through better health care or other factors.

The Metropolitan study found that the overall chances of living to be 100 noware 1,150 in 100,000 - roughly 1 chance in 100. The study termed the chance "very small but not insignificant." Women have a substantially better chance than men - almost 2 in 100 compared to less than 0.5 in 100.

Dr. Charles B. Arnold, who edits the firm's "Statistical Bulletin," which published the reports, said the odds are better than many people may think and represent a dramatic improvement from earlier in the century.

In 1900, for example, the overall odds were only about 30 in 100,000. They rose to 123 in 100,000 by 1940, 199 in 100,000 by 1950, 183 per 100,000 by 1960, and 542 in 100,000 by 1970.

The report says that an exhaustive review of scientific research on longevity failed to show any nutrient, medication, elixir or other substance that promotes an extended life.

But it cites some important geographic variations in the likelihood of becoming a centenarian. Residents of Hawaii, for example, have the best chance, 1,713 in 100,000. Residents of Alaska have the worst, 941 in 100,000.

It lists 10 states with the best chances of surviving to age 100. They are Hawaii, Minnesota, South Dakota, Iowa, Nebraska, North Dakota, Kansas. Florida, Idaho and Arizona. The 10 areas with the worst chances are Alaska, the District of Columbia, Louisiana, Nevada, Pennsylvania, West Virginia, Kentucky, Mississippi, Ohio and New Jersey.

Scientists are not sure why such geographic variations occur. But they have identified a number of factors that may influence longevity of a population, including its ethnic composition; standard of living and income; education level; and adequacy of health care.

The report cites a tendency for extremely old people to exaggerate their ages. Exaggeration is especially common among people living in remote areas of the world that lack adequate birth registration systems.