BOSTON -- We stayed home on New Year's Eve. Okay, I know what you are thinking. How trendy can they get? There she goes, flaunting her life style. Always one step ahead of the crowd.

This is true. Let me add another tidbit. We wore sweatclothes for our New Year's Eve celebration. Yes, yes, call us slaves of fashion. Mine were olive green. My husband's were gray.

Eat your heart out, all you people who went out in black velvet strapless mini-gowns and tuxedos. How many of you discovered to your great chagrin, just as the champagne was crossing your lips, that it was now officially declared ''out'' to go out?

To be frank, I might not have known that I was in the right place, home, at the right time, midnight. We had, after all, been staying in long before it was the in thing to do. But days ago I heard the news on television from a professional trend-watcher.

Faith Popcorn (I did not make up that name) said that in her professional judgment everybody who was anybody will be staying home in 1988. Ensconced there, they will be found eating ''mom'' food, putting on a few pounds, and in general doing the very latest thing trendwise: ''cocooning.''

According to Faith Popcorn, cocooning and home cooking are ''exactly and perfectly on trend.'' In short, it is now hip to hibernate.

You may wonder how this happened. How did the trend curve move right along from cocaine to cocoon? When did the chic switch from sushi bars to home-mashed potatoes? When did the Saturday Night Live set start falling asleep after the 10 o'clock news?

I have a theory about ''cocooning'' which, you will be pleased to know, has absolutely nothing to do with the End of the Reagan Era or the Beginning of the AIDS Era. It has to do with age.

The bulk of the American population, that enormous demographic blip called the baby-boom generation, is at last entering middle age. Tired. At 30, they wanted to have it all. At 40, they are exhausted from trying to do it all.

But alas, no bona fide member of the eternal youth generation can come right out and say publicly that he's suffering from an energy crisis. It's bad enough admitting that you've quit running and taken up power-walking.

I know this because I was born just before the boom, one step ahead of the generation that has lived the stages of its life in neon letters. We got adolescent rebellion. They got the greening of America. We settled down. They got the big chill. We had kids. They returned to family values. The baby-boom generation teethed on its own trendiness. By sheer force of numbers, everything they've done has been heralded as the thing to do. It's been dressed up and marketed in flattering packages that invariably disguise any breath of bad news.

So if baby-boom women are gaining weight, it won't do to call this middle-aged spread. Better to call it the return of femininity. If baby-boom men can't digest nachos and salsa after 8 p.m., let's not chat about emerging ulcers. Better to herald the renaissance of good-ole down-home comfort food.

And if hats and horns and hangovers no longer seem worth the effort, it can't be because of such mundane concerns as babysitters and money. Nor because they've lost the old ability to rebound. That would be too depressing, too down and too downscale.

How much more fun to convince baby-boomers that they are once again on the cutting edge of a massive cultural change.

Are you longing to stay home and go to bed early? Not to worry. It's official now. This is the year of the cocoon. Home is where the hip are.