Not to beat around the bush, my father is a phenomenon. He celebrated his 90th birthday here last week, with about a dozen family members and friends joining him and my mother in a salute to his longevity.

Born in Poland and brought to this country as a boy, he is -- so far as he or we know -- the first member of his family to reach his tenth decade. He has now outlived his father by 25 years -- a goal I will certainly not emulate or achieve.

In that sense, he is part of a larger phenomenon, which is changing the face of America and redefining some of our traditional ways of thinking about ourselves. The num- ber of very old people in the United States is increasing rapidly, and our view of them -- and of ourselves -- is changing just as fast.

The fastest-growing age group in this country is my father's gang -- the over-80s. As the Population Reference Bureau said in a report on this phenomenon last fall, "Since the late 1960s, life expectancy among the eld United States has increased at historically unprecedented rates."

In the seven years between 1974 and 1981, the Social Security Administration's statisticians added about four years to the life expectancy of both men and women. Many experts project that by the year 2000, average Americans can expect to live to the age of 80 -- almost 35 years longer than their grandparents who were around in 1900.

Another -- even more dramatic -- measn Census Bureau projections of the number of centenarians, those who are past 100. In 1982, there were about 32,000 U.S. centenarians. By the year 2000, we can expect to have more than three times as many -- 108,000. And by 2080, they may number 2 million!

The secret behind this extraordinary development is the gain that medicine has managed in the treatment of heart and vascular diseases and, to a lesser extent, cancer. My father, for example, has had a couple of minor strokes; but the doctors and hospitals have learned so much about managing stroke patients that he and my mother are able to live independently, in their own apartment, with only part-time assistance. They are among the lucky elderly, for they are sharing life together after almost 60 years of marriage, rather than experiencing the widowhood that is the condition of a large majority of their contemporaries. They are more typical of the pattern, however, in their decision to retire to the Sun Belt, having moved from Illinois when my father took down his dentist's shingle some 15 years ago. Half the nation's elderly migrants have moved to the 12 Sun Belt states. For the rest of us, including the millions of retirees with living retired parents of their own, the phenomenon of which my father is a part has consequences that are as clear as its blessings.

The extension of human life brings added costs -- for the individual family and for the nation as a whole. As the Population Reference Bureau reported, "Today, the federal government is spending 27 percent of its yearly budget (an estimated $218 billion) on people 65 and older. The proportion could grow to an estimated 50 percent of the federal budget by 2025."

Twice in the last five years, Congress and the president have had to devise "emergency" measures to save the largest single source of income for the elderly, the Social Security system. The Medicare system, which pays many of the health costs for my parents and their contemporaries, is in serious financial difficulty this year.

It is clear that we younger Americans are not going to turn our backs on the elderly or deny them the medical or financial assistance that protects their dignity. As the latest issue of The Wilson Quarterly pointed out in its special section on "The Elderly in America," one of the great success stories of the past two deceen the dramatic reduction in poverty and disease among the elderly. Economist Lester G. Thurow showed that, thanks mainly to government programs, per-capita income for the aged reached a par with the rest of the population in 1970s and, with Medicare and food stamps figured in, actually gives them "a higher per-capita income than the non-elderly."

The growing numbers and political influence of the elderly almost guarantee that those gains will not be wiped out. But it is equally clear that we are just at the beginning of the national debate over the best and most efficient way to finance the programs for the elderonal responsibilities.

There are major controversies about retirement ages, about the options for employment after "normal" retirement, and about the financing and tax treatment of private retirement plans. All these are part of a massive transition from a two-generation to a four-generation society.

No one can pretend these problems are anything but tough. But after last week's celebration with my father, I can't think of a more welcome set of challenges.