Shortly after reaching my 65th birthday, I received a letter from a hearing aid specialist, and one wonders how she knew. I read, "Dear friend . . . loss of hearing can be a terrifying experience . . . it slips away so slowly that you are not aware that your hearing is failing."

The sender urged me to take advantage of "Our special consultation," and included the inducements: "An exciting door prize, the drawing for $100 worth of groceries, special prizes, snacks and a flower for every lady who attends."

Loss of hearing or loss of memory or loss of other faculties are some of the several afflictions that come with advanced years. At 65 one accepts, with some resignation, the few other truths in life are as immutable as the inevitability of the aging process.

My inclination was to write back and tell the hearing aid specialist that as the years roll by virtually everything slips away slowly. But on the outside chance that I may one day take advantage of her offer, and not wanting to blow a chance at $100 worth of groceries, I held my pen.

This was but another of the non-subtle confirmations of what I already knew and for which I needed no further reminders. I have reached my 65th year, and altogether too many people and institutions are taking note of it. I find that I am gradually being made to feel sensitive about my age, a feeling I've not experienced since the '60s when no one over 30 -- even one's parents -- was to be trusted.

Personally I am not aware that I am appreciably different today than I was 10 or even 15 years ago. There may be a change in the intensity rate of my activities but as yet I haven't detected it.

But there must be some outward manifestations of the aging process not apparent to the bearer, but clearly visible to those around me.

Now my son invites me to go fishing or play golf rather than toss around a baseball or football. A daughter sent me a do-it-yourself, stained glass window set. Another daughter gave me a complete set of brushes and acrylic paints with instructions on how to paint seascapes. My wife gave me a dozen golf balls and a subscription to Travel & Leisure.

My grandson sent me a golden yo-yo. He may believe in Aristophanes' definition. "Old age is but a second childhood." But I dismissed the notion realizing my grandson, in his tneder years, is not yet into ancient Greek writers. Instead, I said to myself, "A guy who gets a golden yo-yo from his grandson can't be all that aged and decrepit."

From my colleagues at work, I received a can of tennis balls, a multi-ban radio, and a golf ball on which were printed the instructions, "Call me only for sex or golf."

These are all message gifts, suggestions that it is time to take it easy and enjoy life, you are age 65. These are hardly subtle hints. The messages could not have been clearer had they given me a rocking chair, house slippers and a pipe.

Some wag at a Georgetown disco greeted me with a grin and a wink as I traipsed off the dance floor. "You're looking good, pops," he said. I was probably making a fool of myself out there among the younger generations, but I really didn't care. Vanity, I soon learned, is easy to scuttle at my age.

I had other indications that my age was showing. Returning an overdue book at the public library, I was asked by the desk clerk if I paid fines. "Of course I do, if I have to. What do you mean?" "Well, sir," he answered in a stage whisper, "If you are over 60 you don't have to pay."

"I'll pay, damm it!" I was irked that everyone within hearing distance would know that I was past 60 if I did not pay the fine. I did not feel over 60. On the other hand, I did not know how one was supposed to feel at 60, let alone 65. But I do know I could have whipped that librarian without working up a sweat.And in the next bout I could have whipped the disco clown who called me "pops."

Earlier there was the Golden Age Passport. "Valid, it read right on the back of the card, "for the lifetime of the permitee." I have one of these cards because I got in the wrong line at a National Park Service ticket window to pay for my passage through the park. My I.D. revealed that I was past 62 at the time. My age earned me a card No. 1050041. Egad! There are more than a million of us old folks with free passes to our national parks!

Since then I have learned that I am just one of the 365,000 senior citizens who turned 65 this year; that our numbers grow at the rate of 1,000 per day.

My lawyer advised me that I should update my will and it should be drawn as if I expected to die tomorrow. This is not comforting. With this kind of advice, albeit sound, some morbidity tends to creep into my thoughts.

Most distressing are the daily obituaries. I turn to these each morning with trepidation, always afraid that another acquaintance had died. All too frequently that is the case. So I now accept a truism of my elderstate; the older I get the more funerals I attend.

On meeting an old friend I am now hesitant to ask about a mutual friend for fear of learning that the friend has had a triple arterial by-pass or worse, a one-way pass to necropolis.

For many, 65 is the mandatory retiring age as was my situation, although I opted to retire sooner. I had for several years contemplated retirement with nothing more lofty in mind than to bring down my weight and golf score and do some traveling.

I must recon with the hard fact that I am now 65, that I have reached at least statutory old age. But for my part I have yet to determine what old age really is -- an attitude, a state of mind, a gerontological fact of life, a chronological dictum or a compendium of regulations at each level of the Federal System?

There is considerable evidence that, for better or worse, transition into senior citizenship is largely out of my hands as it is for most of my generation. For instance, take President Carter's budget submission to the Congress for Fiscal Year 1981. Its $155 billion for aid to the elderly (since scaled down) included proposals that would pass me gently through my autumnal years to a dignified burial. I am now embraced by a number of social programs and comforted in the bosom of medicaid which I can take or leave.

So at the very least it is clear my 65th birthday was the signal to federal agencies to move in on me. This was confirmed by my Form SSA 2708 PC.

The form card read "come in" or "call" the Social Security Office. A telephone number was given. All formal enough, but at the bottom of the card was a friendly handwritten note. It read, "Social Security Benefits, I can help you file by phone if you would like to do so." It was signed by a Mrs. Nalls of the Telephone claims office.

I called Mrs. Nalls and, sure enough, she was just as friendly as the note indicated. She explained to me that computer records indicate that soon I would reach age 65 and, therefore, I shuld give some thought to filing for medicaid. Which I did. Now I carry in my wallet, which I have been advised to do, a health insurance card which avers that I am entitled to medicare benefits under the Social Security Act.

And there it was: No less an authority than the Government of the United States had taken note of my entry into the glden years. There could not be more official confirmation of this chronological fact of life than this recognition by the Social Security Administration. I had arrived at 65, an official card-carrying senior citizen.

Sixty-five also means another exemption on state and federal income tax; discounts in some stores, buses and theaters, but not at gasoline stations except for repairs which are discounted 10 percent at some stations. My county governmentsent me a 44-page list of cooperating businesses which offer me discounts on anything from burglar alarms to legal service.

If 65 is another and perhaps last threshold in my life, it must be my passage to the discount generation, that time in my life when I can purchase goods and services at 10 to 15 percent off.

Early in the dinner hour at the Big Boy restaurant I looked around and realized that most of the diners there were senior citizens like myself, even older. There was no floor show. I couldn't buy a hard drink not even wine or beer.

My curiosity piqued, I asked the waitress. "What are all these old folks doing here? Is this some tour group going through?"

"Oh no! They're all local. They are here for the Golden-Ager Deals."

"What's that?"

"It's the senior citizen special. Right there on the bottom of the menu." And there it was, two pieces of crisp fried chicken, hash brown potatoes, peas, rolls and butter, all for $3.09. With this, at no additional cost, all one can carry away from the salad bar. That alone, for the roughage addicts, was worth it.

"In that case, I'll have the special." I did not have to produce my Senior Citizen I.D. card issued to me by the county's Community Affairs Office. At a glance, the waitress pegged me for a senior citizen. Again my age was showing.

It was the third time this day that I had occasion to flash my newly acquired credentials. At a local pharmacy I asked the sales clerk if this drugstore participated in the county's Discount Program for Senior Citizens. It did. The result: I saved 55 cents on a $5.50 purchase.

I paid half-price for two seats at the Kennedy Center. I traveled from place to place at half the normal cost with my Metro Discount Card issued by the Washington Metropolitan Area Transit Authority.

By my rough calculations I saved about $17 on $40 worth of purchases. Not a bad day. Had I played a round of golf at one of the public golf courses, and had my hair trimmed (which I do less frequently these days), I could have saved another $5.

These are the few perquisites on reaching 65. For many on fixed or limited incomes these savings are critical. A 20 percent discount at a podiatrist who participates in the program is a great aid, because one of the afflictions of old age, as I am now painfully aware, is foot trouble.

At 65 I learned quickly that old age has its own apocalypse -- sickness, loneliness, poverty and senility. I have seen these in the waiting rooms of doctors' offices, at the Social Security Office, Senior Citizen Centers, and convalescent homes.

I had read that the elderly fear senility more than death. That's probably true. I fear senility, probably not senility per se, but a debilitating disease that will strip me of my independence -- a crippling disease or accident that would deny me the freedom to move about under my own power, the freedom in knowing I do not have to depend on someone to wheel me about or to shop for me or to feed me or -- even worse -- to bathe and dress me.

Yet as I read about the conditions of my age group, I became concerned about how little our doctors know of geriatric medicine. American doctors are not interested in this area of medicine and get little training in the subject in our medical schools.

It is also disturbing to learn that at my age I may appear to hold my liquor better than a younger man, but in fact my memory and decision-making equipment will suffer more quickly than those of younger people. It is not at all comforting, then, at age 65 -- now free to take a snort whenever I feel like it -- to learn that the drug, alcohol, is more risky for me now than a few years back.

Nor is it a comforting thought to learn that what is a safe dosage of prescribed drugs for a younger man my size may be an overdose for me, probably resulting in falls, fractures and periods of confusion (at which time someone will hang a sign on me that reads "senile" when, in fact, I've simply O.D.'d).

I had this unfortunate experience, and it had the effect of spoiling a charming hostess' cocktail party. One scotch and water and I literally slid under the buffet table. Earlier in the day I had taken a prescribed muscle relaxer and the combination did me in. I have not been invited back to that home.

I am also forewarned that inactivity can be as devastating as disease. It may even be regarded as senility. The boredom penetrates the heart and soul and from there the degenerative process begins on the human will. Without the will to live -- what the hell!

My strong survival instincts drove me back to work, and work drove me back to leisure. But in my new-found leisure I have learned to use the paints my daughter sent me, I have gone through the golf and tennis balls, learned how to use the super savers for travel, and the golden yo-yo, and involved myself in community and political affairs.

I have befriended a backyard squirrel who eats white bread out of my hand. I am now able to identify the eight species of birds that nest on my property. Also, I can now distinguish between the fragrance of a rose and the bouquet of lilac.

As if an ultimate confirmation of having achieved the beginning of my golden years, I received an invitation to join the 50-90 Club, but on learning what the name really means, I gracefully declined. Somehow I would rather be "Pops," making a fool of myself at a Georgetown disco, than doing arts and crafts and dancing with octogenarians at the Local Community Center. I'm not ready for that.

Now I am working on a "kind of happy nature" to replace my sometimes caustic and irascible one. The Greek philosopher, Plato, who reached 80, wrote, "He who is of a kind and happy nature will hardly feel the pressures of old age . . ." I've got to go with Plato.