LET OTHERS prattle of unfulfilled dreams, graying hair and shifting relationships. Miss Manners alone knows the true perils that beset the Woman of a Certain Age.

A sensible woman is generally equipped to handle the normal vicissitudes of life, although a little gentle moaning is occasionally allowed, in the privacy of her boudoir. What she did not expect was to be caught, in what ought to be the graceful years, smack in the middle of an etiquette revolution, with wild men going off in all directions around her.

The revolution, not yet completed, is the shift from a system of precedence based on gender to one that shows signs of eventually using age and rank as the basis of precedence. We cannot all go through the door at the same time, as an astute diplomat once remarked, and general agreement on who should go first makes for a peaceful life.

But the change is by no means complete, and while most of the society is trying to squeeze through that door simultaneously on the charming system of "me first," the lady is left standing there.

Does she wait, as she was taught, for the gentleman to open the door for her? She may very well be left waiting while he saunters down the block. Does she plunge ahead and open it herself? Then she leaves behind a pouting gentleman who had been planning to get to the door handle, as he believes is proper, just as soon as he could figure out on which side it was.

Mind you, this lady was described by Miss Manners as a sensible lady, so she is only trying to go along with prevailing manners, not to enact her politics through symbolic behavior, any more than she wears her philosophy printed across her bodice. But she can't figure out what the prevailing manners are, since her acquaintance includes young gentlemen who were never taught ladies-first courtesy, older gentlemen who have renounced it, older gentlemen who practice it, and younger gentlemen who have recently taken it up because they have noticed how quaintly effective it can be.

Ladies do not wish to imply disapproval of gentlemen's manners when the gentlemen plainly intend to be mannerly, so they must develop a few agile and ambiguous techniques.

At the door, for example, the lady can turn and smile at the gentleman just as she reaches the handle; she can observe, then, if his hand is out.

If it is a revolving door, she steps in, waits a split second (unless it is already going around, in which case she would be flattened like a fly by a fly swatter--but we were talking of sensible ladies) and then pushes with her arms unbent, so that her propelling hands are not visible to a gentleman who is gearing up his strength to push for her.

When walking down the street, she gains the inside on the pretense of looking into the shop windows, not because she seems to expect to be so placed.

When about to alight from a car, she waits momentarily to see if the gentleman seems to be coming around to her side to open the door. The sight of his back, entering the restaurant, indicates that he is not, so she opens it herself.

In the restaurant, she orders her meal with her eyes cast down on the menu and her face halfway between the gentleman's and the waiter's. If the gentleman wishes to repeat the order, he may; if not, the waiter has heard it.

You see the principle. It is troublesome, not to mention hazardous, and Miss Manners is not unsympathetic to the ladies who impatiently decide to adopt the new system for all, expecting no precedence--provided they do not get unpleasant when it is offered.

But manners are folk custom, and we are most charmed with what we know in childhood, which is one reason for practicing the traditions among those who enjoy them. Another is that all this jumping about is good exercise for ladies of a certain age who do not care to jog. MISS MANNERS RESPONDS

Q. My husband and I became engaged while he was on a university assistantship. My engagement ring contains two pave' diamonds, at his insistence.

My husband has little inherited wealth; his cousin, however, has a lot. His cousin's wife, coincidentally, wears an engagement ring containing as many one-carat stones as her husband's father has brothers. My husband is maternally related to them.

One evening, after a family dinner hosted by this cousin, his wife turned to me and exclaimed, "Is that your engagement ring? It has diamonds, hasn't it? But . . . where are they?"

I realize this is a sad situation to be concerned with, but I felt my husband and his family had been the intended recipients of her remarks. Although she inquires about the authenticity and price, and insists on examining all my jewelry customarily, this was the only moment in which I sensed her to have shown shrewder than usual insight into the gentle art of making enemies, for, after rigorous scrutiny of the ring, she professed to have been unable to locate the stone without optical assistance, and we parted in search of a magnifying glass, which remained as undiscovered as the diamonds for the rest of the evening.

Must I resort to literary revenge in yet another scathing expose of the middle class, or could you suggest a more agreeable way to approach this situation?

A. Let us not blame the entire middle class for this. Miss Manners, for one, is weary of hearing the middle class picked on, especially by its own members.

However, she is not adverse to picking on someone of your husband's cousin's wife's kind. What is more, she is willing to tell you exactly how to do it, eschewing cheap generalizations and rudeness. It is one of Miss Manners' basic tenets that there is nothing wrong with defending oneself from the bad manners of others, provided one does not use the same approach in doing so.

Here goes. You may alter the details, if you wish, to fit the facts of your life.

"Ah, you asked about my engagement ring. You can't imagine how much pleasure it brings me--let me tell you the story.

"You remember that Charles was still on his university assistantship when we became engaged. We were so young and so poor that we knew we should have waited to get married, but we were so much in love, we couldn't, and, as it turned out, those years of struggle are so precious to us. Anyway, I've never much cared for flashy jewelry, and I never dreamed he would be so foolish as to buy me an engagement ring, but Charles is so romantic that he absolutely insisted, and he wanted it to have diamonds in it. Even now, my eyes fill up when I think what he must have sacrificed to get it. I wouldn't trade it for all the queen's crown jewels.

"By the way, that's a very impressive ring you have. Were you ever a jeweler or a pawnbroker or anything professional like that? You have such an exact eye for appraising things. How you must be amused by a sentimental idiot like me.