All About Erica : If Erica Jong were a truly rebellious spirit, she wouldn’t need to remind us of it so often : FEAR OF FIFTY: A Midlife Memoir, <i> By Erica Jong</i> ; <i> (HarperCollins: $24; 320 pp.)</i>

If it comes as a bit of a shock to self-proclaimed bombshell Erica Jong to walk into a room and suddenly find that she is no longer the “youngest” or the “cutest” there, she certainly has our sympathy. Yes, these things can happen, to the blessed, the talented, even to the thinking person. Then again, superstars have always found ways to cope with the dismaying process known as aging. For the desperate, there is always cosmetic surgery, the spa, an athletic trainer, a special diet; others, alas, like Marilyn Monroe, may resort to an even more radical solution.

Jong’s current title harks back to her first, fearless smash in 1973, “Fear of Flying,” in which she coined an unquotable phrase about a “zipless” form of pleasuring she designated as the “symbol of my generation’s hunger for female freedom through sexual freedom.” As for her latest, “Fear of Fifty,” she has appended to it the subtitle, “A Midlife Memoir,” thus protecting the book from the rather specific rules governing autobiography; namely, that one must begin at the beginning, “I was born in . . . ,” and progress from there. That explains why, from the outset, one is met by Jong’s faux-naive reaction to the closing of her fifth decade of life--her discovery of gradual decay in the carnal self. It would seem, Mr. De Mille, that she does not exactly relish her next close-up.

Though she willingly admits she “will never be Madonna or Tina Brown or Julia Roberts,” she regrets much more that by the time this book comes out she will no longer even be considered for “the flavor of the month.”

Apparently the statistics get her down: “Every year another crop of beauties assaults me on the streets of New York. With thinner waists and blonder hair and straighter teeth, with more energy to compete (and less cynicism about the world), the class of 1994, or 1984, 1974, is inexorably replacing my class Barnard ‘63--yikes!”

So her real trouble is that her attitude to turning 50 resembles the ingenue stance of her salad days. For about 20 years now, since her first novel, the feisty, competitive, foulmouthed, confessional shtick has worked admirably for this writer; it carried her to fame and fortune. Why not shoot into midlife on the same wave?

The trick might just have worked too, at least as fiction--a midlife heroine’s outrageous romp around (gulp!) the clock. But, when it comes to autobiography ( good autobiography at least), there are, unfortunately, some essential requisites. First off, autobiography needs more from a confessor than her simply being “famous for being famous,” as the historian, Daniel Boorstin has phrased it. Then again, the genre inevitably invites comparisons with superior minds. We need to know, for example, why the detailed, intimate recitation of a certain woman’s life might engage? And why at this time? Will it reveal insights into her special genius, her achievement in the literary art?

One is reminded of the late Anthony Burgess’ story of his early life, “Big Wilson and Little God,” or Mary McCarthy’s “Memoirs of a Catholic Girlhood,” two memorable voices, both for their evocation of their artistic careers and their honesty about their own petty failings. Jong’s tale clearly does not avoid the latter. Indeed, hers is a remarkable candor; nor can she be faulted for not telling all. The question is: Does she exhibit more than we really care to see?

With Burgess or McCarthy, we welcome the revelation of gaping faults behind their brilliant constructs and imaginative creations because they teach us what their humanity is like. But Jong’s confessionals, on the contrary, appear more like a preemptive strike against oblivion.

Not that there aren’t some lively reminiscences to be found in “Fear of Fifty.” Several of Jong’s takes--for example, her tender recollections of her Bohemian Jewish family, her relationships with her sisters and grandparents, her indulged Manhattan childhood, her delicate first loves--are touching, and seem true to her notion of herself.

In such chapters as “The Mad Lesbian in the Attic,” about her old aunt’s desertion by her lover of 30 years as she succumbed to dementia and Alzheimer’s, and “Baby, Baby, Baby,” about the birth of her beloved daughter, she sounds genuine in her emotions.

Most affecting, perhaps, is her reprise of her affair with her latest husband, who seems literally and symbolically to have cured her of her long-held fear of flying. A passionate pilot who is transfigured each time he is airborne, and is eager to open such experience up to the fearful Jong, he clearly becomes the perfect hero for Jong at 50, with herself so eager to be “free of her body.”

And there’s her interview with her father, in which she challenges him with, “You’d still like me to be a doctor, wouldn’t you?” and he answers with compliments about how fine a writer she really is, and how her only problem is bad PR: “Look at Madonna. She’s got no talent, but great PR. Why don’t you call that Della Femina guy. He’ll advise you.” This is delightful too.

What follows, however, is an editorial commentary she repeats after most of the charming narratives in the book, and it reads like a “making it” whine:

“How can I explain to him that the vicissitudes of my career cannot be undone by mere ‘PR.’ I have broken rules that are invisible to him because he is a man: written openly about sex, appropriated male picaresque adventures for women, poked fun at the sacred cows of our society. I have lived as I chose, married, divorced, remarried, divorced, remarried again--and still worse dared to write about my ex-husbands!”

And with this comes her self-exculpatory conclusion that hers is “the fate of rebellious women. They used to stone us in the marketplace. In a way, they still do.” It begins to seem as if this is the pathetic refrain of her life.

Mistake not. Jong’s problem is not in having written about her (mis)adventures but in presenting them as though they were the vital news, the real poop, and right for everywoman, instead of mere cliches formulating what Jong defines as the sexy, the liberated, the feminist life of courage.

A sample of some of her profundities:

“Married men are, of course the best lovers, unless you happen to be married to them.”

“Divorce is my generation’s coming of age ceremony--a ritual scarring.”

“Dead mothers are rather easier for teenage girls to honor than live ones.”

“Conversation. It fills a lot of life’s little gaps.”

And perhaps the deepest, most Jongian, of all: “You don’t get to choose what you get famous for and you don’t get to control which of your life’s many struggles gets to stand for you.”

No kidding!