"Middle Age Crazy," in which Bruce Dern gets tired of Ann-Margaret but not for long, is a fairy tale for 40-year-olds. The most effective fairy tales, as any child knows, have terrifying gory endings. That "Middle Age Crazy" does not is alas, but not necessarily a lack. Bobby Lee Burnett, the middle-age crazy hero, is a successful builder of taco stands in Texas who, at the drop of his 40th birthday, leaves his wife for an interlude with a Dallas Cowboy cheerleader. Bobby Lee then suffers the pangs of non-virture for about one week before hastening chastened back to the marital fold.

Despite that curiously pat resolution, "Middle Age Crazy," which opens today at 11 area theaters,is a good-spirited film which strives mightily to reproduce life among grown-ups. For Dern, and somewhat less so for Ann-Margret, the roles of Bobby Lee and his "dumb-bunny" wife Sue Ann are an opportunity to celebrate their own career passages.

Dern began in movies by killing John Wayne in "The Cowboy," and has since embodied weirdos at work from outer space (the antisocial gardner of "Silent Running") to the Super Bowl ("Black Sunday") to the shell-shocked husband in "Coming Home." But Dern has let it be known that after 36 films he is tired of playing psychopaths. Dern even told Dick Cavett how disappointed he was not to get the Jon Voight role in "Coming Home." Food for thought, eh?

Thanks to the new Bruce, Bobby Lee Burnett is good-looking non-lunatic whose eyes never take on the historic Dern menace. Bobby Lee isn't nuts, he's just having a birthday: "The Big 4-0." In attaining this responsible age, he has also attained a closet full of three-piece gray suits, an I'm OK-You're OK Oldsmobile, a company full of loyal taco-stand builders and a family life that is perfect right down to the bubbles in the Jacuzzi.

What's right, however, is also what's wrong. Tortured (in a non-pathological way) by the knowledge that he has no reason for unhappiness, Bobby Lee sets out to screw things up good. When his middle-class brain turns to mush after two glances at an off-duty Dallas cheerleader, he quickly pruchases the most expensive Porsche inthe car lot, a cowboy shirt and a ticket to moral bankruptcy. It is, however, a two-way ticket.

While Dern gets a real role (as he did in "Smile" and the "The Great Gatsby"), Ann-Margret gets to play another love-crazed yes-girl trapped in a world she never made. As Sue Ann, she virtually smothers Bobby Leein Texas-scale smoochola, to which he reacts like Lord Mountbatten being offered command of the Love Boat.

Sue Ann is a clinging vine with thorns, and the crowning thorn is the Betamax she presents to him on his 40 th birthday. One by one, Bobby Lee's responsibilitites greet him on the large screen: Sue Ann, tears of love in her eyes, arms extended like an octopus; his son, pledging allegiance; his mother filled with pride; even his son's girlfriend, an uncomfortably numbile reminder of Bobby Lee's present distance from youth. Only his father, retired and not liking it much, sounds a note of caution. What's wrong with being 40, grandpa asks from the screen. It's got to be better than 63, because I'm 63, and it's the s----."

Bobby Lee begins to have daydreams. In one of them, he tells the graduating senior class of the local high school not to grow up. "Stay 18 for the rest of your lives," he counsels. He knows as well as anyone else that Peter Pan in a three-piece polyester suit is ridiculous, but he can't help himself. And when his father dies, the middle-age trap closes tighter. He's the daddy now, he learns. "But I don't want to be a daddy."

Before you know it, Bobby Lee has fled to the arms of his cheerleader. Shacked up in Houston and missing business appointments right and left, he pretty soon feels uneasy -- queasy about his new course. His wife is mad as hell, his cheerleader turns out to have several other cowboys (O despond!) and, besides, you have to bend like a pretzel to get in and out of a Porsche. As an affair to remember, this one is destined to be mostly memory. He soons goes back to hearth and home, or in this case, Ann-Margret and Jacuzzi.

This is supposed to be funny, but it plays a little like social commentary, partly because Bruce Dern isn't Burt Reynolds, and can't help looking sincere most of the time. In fact, little of the material in "Middle Age Crazy" is designed for belly laughs, but rather to touch some universal experience associated with middle-age passage-making. Now that Gail Sheehy has become the Dr. Spock of adulthood, we're apparently fated to encounter a new kind of puberty every 18 months for the rest of our lives.

As a fairy tale, "Middle Age Crazy" is fairly harmless, but as proof that Bruce Dern isn't crazy after all, it works fine. After all, he does go back to Ann-Margret in the end.