"Oh you're not old" is the instinctive response if you are getting along in years and allude to the fact. It's as though you are being assured "you're not dirty." But if you are not "old," then what are you? You are "elderly," you are "a senior citizen," you are --heaven forgive us--"mature," an honorable word that used to be reserved for those of all ages who earned it. Occasionally, someone is "older."

Has our language been robbed of a decent word? "Old" still has its uses. We are permitted old wine, old silver and china, old carpets and old furniture, to all of which age still adds value. And we can enjoy old pets, which suggest love. The ancient and honorable word has but one human use: for the poor. Lacking so much else, they have at least that possession.

In his "New Dictionary of Quotations," published when he was 62, H. L. Mencken tells that Horace, just before he died at 57, lamented that "Waning years steal from us our pleasures one by one; they have already snatched away my jokes, my loves, my revellings and my play."

At 41, Walt Whitman sang of the "grandeur and exquisiteness of old age." At 71, he cried, "Here I am . . . much like some hardcased, delapidated, grim, ancient shellfish or time'bangd conch cast up high and dry on the shore sands."

Since Mencken, life expectancy has grown considerably, chiefly among females. A crude reminder greeted a shipload of American tourists not long ago. Arriving in Manila Bay, they rated a front-page story: "Yesterday the SS Carolina docked in Manila. On board are six hundred American widows whose husbands died of heart attacks while earning the money to make their trip possible."

The idea that old people might become a serious public responsibility and an attractive market for private enterprise dawned in the '30s and burgeoned in the '60s. In the public sector, it created Social Security and Medicare; it shared in poverty programs, such as welfare and food stamps. In the private area, it spawned national membership organizations, old age communities, insurance schemes, small "senior citizen" privileges, countless group tours, television programs. A third White House conference on aging opens in Washington today.

A whole literature is targeted at the "elderly," specifically at the well-heeled. One series of pamphlets is called "Action for Independent Maturity." Start with You and Your new retirement home. Next, You and Your money, You and Your health, You and Your social life--elderly gentlemen have "friends," elderly ladies "dates." Finally, You and Your funeral. These publications block your view of the outside world. There is only one place to look-- inward. Is this what you want?

Simone de Beauvoir disposes of this question in "The Coming of Age." She recommends "a fairly committed, fairly justified life so that one may go on in the same path even when all illusions have vanished and one's zeal for life has died away."

To warn against an obsession with personal concerns is not to suggest that money is a bad thing. Millions of old people slide into real poverty when they "retire." Must they retire? We Americans are a working people. Leisure has not been a part of our basic culture. Can one afford to retire? Will one become a burden? What would one do?

Do I hate my work? If so, I'll try and change it. What are the chances of getting another job at 60? Forced retirement, with or without pensions, legal at 70, is spreading. Its victims find they have less money, less status, less interest in life. Taxpayers are discovering that they will have to support more and more old people who could be working. When irate stockholders rally against forced retirement, it will be good news.

Must the old be tossed into idleness and poverty so that young people can work? "It is simply not true that there is not enough work in the United States," writes Dr. Robert N. Butler, a leading social scientist, in his excellent book, "Why Survive?" "The truth is that our need for goods and social services requires an expanded work force." Butler is for "loosening up our lives," now rigidly programmed into education for youth, work for middle life, idleness for old age.

Another hold on life, perhaps even more essential to a tolerable old age, is love. But as families disperse, old people have less chance of remaining physically and emotionally involved with their own kin. And so it is perhaps lucky for the old that they need to love, even more than they need to be loved. Work and love keep alive one's affinity with the human race.

Leonard Woolf, 89, spoke for many of us: "I cannot disengage myself from the real world; I cannot completely resign myself to fate. It is in the pit of my stomach as well as in the cooler regions of my brain that I feel and think about what I see happening in the human ant-heap around me, the historical and political events that seem to me to make the difference between a good life and a bad, between civilization and barbarism."