THERE I WAS ON AUGUST 31, 1990, contemplating the end of summer, the imminence of autumn and some other matters, when I spied this front-page teaser in the Wall Street Journal: "Nintendo campaign targets men age 18 to 49, B4." I could not bring myself to turn to the specified page. The headline said it all. I had just five months to play Nintendo.

Of course, for some time (the last 10 years) I've been aware that I'm coming up on the half-century mark. I approach 50, as I do everything else, with equanimity and with, I may as well add, the profound hope that I do not sink into a deep depression. But what did not occur to me until I glanced at the headline was that this birthday is going to be worse than 40, which, in case you're interested, was bad enough. I will soon be going from one market segment to another.

You see, in lore as well as reality, 40 might be just the right age for a midlife crisis, but in the minds of many marketing mavens, you remain in the same age cohort. On Madison Avenue and elsewhere in advertising-marketing and other forms of fibbing, a 40-year-old is no different from a 22-year-old or a 36-year-old. They are all treated as one age cohort, one market segment -- one grand group of acquisitive people off on a grand buying spree: running shoes, tennis rackets, barbells, tight jeans, Club Med vacations, little sports cars with big engines, condos, town houses and anything else that can be bought with a credit card and the belief, which comes with youth, that these items will bring happiness, when, in fact, they bring nothing but indebtedness -- a different feeling entirely.

This, then, is the market segment that I am fast leaving. I could say, "And good riddance too," but that phrase would be more bravado than truth, a kind of whistling past the graveyard. There are aspects of my market segment that I won't miss, of course. Some of the time I found it inane, mindlessly youthful, too obsessed with music and things that were, increasingly, tight on me (shirts, jeans) and, if truth be said, repetitious. The thing of it is that as you get to the absolute border of a market segment, as you are about to leave one and enter another, you find magazines repeating themes that no longer have any meaning or touting celebrities that experience has taught you will soon be gone. I remember when Phyllis George was the future of television.

But whatever my qualms with my age cohort, whatever my troubles with my market segment, I knew this: I was wanted. To be a member of that 18 to 49 group was like being in the right Zip code and always being told how wonderful you are. You got letters from the finest stores telling you how smart you were to live in the best area in town -- in other words, how smart you were to be rich -- or how only special people like you would receive the highly glossed and extremely dumb magazine that had been sent to everyone in your Zip code. It meant that in filling out some questionnaire, you could put a check next to the market segment that was the most desirable, that everyone wanted. And if you made more than, say, $50,000 a year and were, say, 30 years old, you were simply golden. Everyone wanted you -- even the Nintendo people, may they rot in hell and go bust by February.

And there was something else as well. As long as you were in the 18-to-49 market segment, you were young. In some ways (don't ask me to explain), you were 18. I mean, if the dumb marketeers didn't distinguish, why should you? They set up this category as if everyone in it was the same. Eighteen to 49, they said -- as if we were all supposed to read the same magazine and play the same games and (would you believe?) get into the same jeans. It was, like, awesomely stupid, but I liked it.

But in a mere four months or so, I will go from 18 to 50. I am now about to join the nightly news cohort, the people who watch Dan and Tom and Peter, apparently while awaiting death. They have aches and pains, not to mention constipation and false teeth. I know this from the commercials. It hardly matters to the marketeers of this world that I strongly feel I'm not ready for that age cohort, that I feel fine and that, really, my teeth are my own. They will drag me, kicking and screaming, into the next market segment. I do not want to go.

I am not sure how one moves from one market segment to another. I suppose you check the age box on one of those questionnaires and it just happens. Suddenly, certain magazines stop coming in the mail, and certain commercials, for racy cars or Club Med vacations, say, get jammed so your television cannot receive them. I imagine I'll get "carded" when I try to buy something at the Gap. "Sorry," the clerk will say, "you're over-age." All I know is that just one day before my friend Pringle turned 50, he got in the mail a membership application from the American Association of Retired Persons. The very next day, he was out of one market segment and into another.

But the thought occurs to me that while I have to forsake Nintendo (a game linked with the devil, communism and the black plague), I will be, for the first time in my life, in the same market segment as my parents and some other pretty terrific people. We can share the interests of our market segment, which, I know from the commercials, include gardening and painting with arthritis. One thing for sure: We won't be playing Nintendo. If they don't want us, we don't want them. Besides, Nintendo makes your false teeth fall out.