WHEN EVEN THE cliches change, you know it's time for someone to start writing a book.

Just yesterday, it was kids dressing up like Mom and Dad, teetering on Minnie Mouse heels, wearing pants that collected around the ankles. Today it's the executive with fried eyes who wears the medallions on his gray chest and talks about scoring kilos.

Just yesterday, the bars were full of puffy old guys talking to their martinis. Today the bar is a sweet shoppe, drinks come with cream and sugar, and everyone is trying to make a date.

Just yesterday, smoking in a high school boys' room was a hanging offense. Today the ads show us our ever-laughing young jauntily cigaretted while waterskiing, sailing or climbing.

Just yesterday, dance bands were peopled by bald men with rimless glasses. Today you get some kid with a safety pin in his nose.

Cars too: designed for the young and uninsurable. Movies. Magazines. Shoes. It's called the Youthquake. You're expected to pay attention. If you're not part of it, you're supposed to wish you were.

But as David Riesman would say, you don't have to be who you're supposed to be, there's no reason why you should listen to those Others who would direct you, and in fact there are quite a lot of people who get along perfectly well being themselves, and that's us.

A memo from the Invisible Generation:

We are the people who run the world. We own the most property, earn the biggest salaries, pay the highest taxes. We dominate the ruling bodies of the globe. More than most, we know how to cope with chaos in our lives. We are stable and at the same time adventurous, sound yet impulsive, worldly yet visionary. We are not depressed by that depressing reflection, "If youth knew; if age could," because we know and we can. Pretty much.

"We're in the prime. It's the most delightful time of life. You've made your mistakes with your children and come to a peace with them, and you've established some kind of proficiency in your profession and are going ahead with that. The ones we hear about in the news are the ones who can't cope. But there is a large body of us who do."

So says Elsa Porter, Assistant Secretary of Commerce and a survivor of motherhood and a live person.

Our name is Middle-aged, and in America that is not good. Sometimes we are patronized as "mature," but no sooner did some helpful euphemist apply that word to us than it became an insult. In truth, we are not patronized to our face. We are too powerful.A humorist has called our condition "middlescence," and there are those today who study "mediatrics." But we will stick with the original putdown: middleaged.

So what I want to know is: Why don't you see us on the TV commercials? Why do the pop philosophers write about youth and age but almost never about the yawning gap between?

"You're right, there has been practically no serious literature on middle age until recently," said Dr. Robert N. Butler, director of the National Institute on Aging and a Pulitzer-winning author on the subject.

"However, in the last few years there's been a fair amount. After all, this is an extraordinarily exciting period in life, a time for major reexamination of personal commitments, for redirection in lifestyles and occupations. It's also a time when people can feel burned out or bored with themselves, when you see alcoholism, obesity, addiction to pills, divorce and general confusion."

Butler, who is 51, cautioned against simple conclusions. The empty nest syndrome, for instance, he finds is by no means universal. "Many parents are ecstatic when the kids leave home. It means they can be free to do what they want again."

Perhaps, he suggested, the very complexity of the problems and the immense variety of solutions are reasons why the pop media don't want to tackle middle age. "Not only do you have residual problems not worked out from youth, but you have the new problem of coping with death."

So if this is the age when, as Butler quoted Schopenhauer, a person stops counting forward from birth and starts counting backward from death, the time when, as sociologist Michael Maccoby observes, the survivors "are the ones who have committed themselves to something beyond just winning games," if these people are so exciting and so competent to cope, then why are we so hard to find in America's image of itself?

Maybe we are anathema to Child America because we are its parents. The young can be in worse shape than we are, with their junk-food diets, but it doesn't count. The very old are considered cute. Grandparents are, after all, allied with grandchildren against that grumpy generation in the middle.

You want some facts? Check the TV ads for an hour at random:

A rug cleaner commercial shows a young family with small children; for analgesic pills we get a tiny drama about teething babies keeping a young mother awake at night; a hamburger ad gives us happy 10-year-olds.

And more: scouring pads, housewives; soap powder, housewives; power company, young hardhats; canned ham, a granny; paper towels, a housewife; cleanser, two housewives; fast food, teen-agers; toy trucks and campers, kids; chewing gum, teenagers; television devices, a young family; frozen orange juice, mother and kids; stereo, teen-agers; credit cards, a young athlete; cosmetics, young women; hair grease, young men and women.

There is indeed a bank president, but he's a cartoon with a self-satirizing voice like the Great Gildersleeve and thus nonthreatening. There are a few local car dealers, definitely middleaged and definitely talking to middleaged viewers, but what you see mostly is cars: the dealers are visible only for a few seconds, the putative customers not at all.

Even life insurance ads go heavy on the bright-eyed, curly-haired young beneficiaries.

What few middle-aged characters appear in network commercials are romantic figures, grizzled cowboys, seamen, outdoor adventurers safely removed from suburbia.

Sociologist Amitai Etzioni calls us the Sandwich Generation. "When the children are out of college you think life's going to be easy, but they get married and divorced and come back home, or they park their grandchildren on you, or they lose their job and want their old room back, or they decide to go after a PhD. All this time you're also caring for your own parents. You're hit on both sides."

Television, he added, doesn't even bother to collect statistics on older people.

There are some figures, however. Out of a total network advertising budget of $2.3 billion in 1975, over $400 million was spent on specifically childoriented ads such as breakfast novelties and toys. By high school graduation time, the average American has logged 15,000 hours watching the box, including 350,000 commercials. (Also 18,000 murders.)

Obviously, the youth market is a great market. But look at this: People of 55 and older consume 41 percent of the coffee, 42 percent of the tobacco, 37 percent of over-counter drugs, 35 percent of lawn and garden items, 32 percent of paper products for the household. They spend one dollar out of every four spent on cosmetics and bath products. And so on.

These Department of Labor figures were provided by Ernest Dichter, the Wizard of Motivational Research, who revolutionized advertising with his ideas about the subliminal and the dream-wish.

Asked for comment, he shot back this remarkable memo, quoted as he wrote it:

"Parents represent not just an example but a challenge. I have to listen to them in real life. Don't use them in ads or other public pronouncements. It aggravates the pressure. Advertisers use young people often simply because they have inherited the knowledge that everybody likes young people. Middle-aged people indeed do admire youth but they are jealous....

"This is not for me, is often the reaction when beautiful women are being shown. Most of us learn something as the years pile up if nothing more at least to be less trustful, to accept the fact that things don't always turn out exactly as we planned. We wish we could make the younger population aware of how quickly time passes, how easy it is to make wrong plans.

"Beginning maturity involves investment in yourself. Not just to buy products but to buy products which permit me to develop new interests and skills.

"Many of the ideas we have about older people simply are not true. They are not governed by habits. If anything they are more interested in making a fresh start, getting a second chance."

Is there a conspiracy on TV to make us invisible?

"Probably not," says Larry Light, director of network programming at Batten, Barton, Durstine and Osborne. "It's not all that complicated. We just use the most effective way to get the message over. We don't cast any of our ads based on the people who use the products. We tend to appeal to people's minds and hearts. Like a soft drink, like the Pepsi Generation -- when people see the ad, they see something in their mind's eye."

One of the biggest mistakes made by TV advertisers, he suspects, is to insist that the consumer of a product be the hero of the commercial. When a woman watches a cosmetic ad, she doesn't see herself up there, she sees the beautiful model. She doesn't buy a product; she buys a hope.

"In this culture it's not good to position the product as being for old people. It's not a neat idea."

Never mind. One of these days the invisible audience will be too big to ignore. The median age of Americans, which was around 16 years in 1790, is close to 30 now and will go above that in another three years. By 2030, one American in six will be over 65. Already there are 23 million of these, and another 23 million people between 35 and 44. There will be nearly twice as many of them by the turn of the century.

The average age of members of Congress is 51. So there.

These days it is pleasant for us to think about the myth of Icarus and Daedalus, the father-and-son team who built wings for themselves to fly out of prison. Icarus, the world's firt hot-dog pilot, immediately flew too high and his wings melted in the sun. But his old dad, the old artificer as James Joyce called him, the craftsman who invented the wings, the practical dreamer who had the vision of flight in the first place, flew steadily to freedom.

There to join the vast secret society of the quietly comptetent, the invisible movers.