In an age when Viagra has revolutionized the sex lives of older men, who would have thought anyone would take issue with Gail Sheehy's contention, in her new book, "Sex and the Seasoned Woman," that aging women want lively sex lives too? After all, didn't women of a certain age grow up in the same swinging '60s and '70s? Don't baby boomers believe they invented modern sex?

And who are all the millions of Viagra-fueled seventysomething men supposed to have sex with — women in their 20s?

Essentially, yes, say Sheehy's critics, who happen to be maturing women themselves.

Daphne Merkin, who famously wrote in the New Yorker about how the erotic pleasures of a good spanking led to love, marriage and motherhood, opined in the New York Times Magazine that Sheehy's dismissal of the idea that "menopause signals the end of a woman's gainful experience" fails to deal with what Merkin views as male disinterest in older women.

"It would seem fairly self-evident that as women enjoy longer and more active lives in a culture that venerates youth, especially in women, something's gotta give — and what gives, mostly, are men," she wrote. "Men of 45 aren't looking for women of 45; men of 55 aren't looking for them, either. Nor, apparently, do you have to be Jack Nicholson playing a version of himself — a rich, insanely charming Don Juan — to think that you deserve a spring chicken on your arm."

As anecdotal evidence, she offered a 50-year-old friend of hers who was seeking a woman in her late 20s, noting that "a woman this age would come with a guarantee that her eggs were fresh."

"My book is a Rorschach test," Sheehy retorted on the Huffington Post. "Women who belong to the glass-half-empty crowd seem to find threatening my reportage on women 50 and over who refuse to accept that they're over — but instead are ... enjoying a resurgence of desire for romance and sex, and actively pursuing new dreams and passions to enliven the many decades they have ahead."

And in spite of the marketing-friendly title, it's not just about sex, Sheehy, herself a "seasoned woman" of 68, explained as she relaxed recently on a leather sofa at a house in West Los Angeles.

For the tsunami of female boomers pushing past 50, it's also about power, and options, and pursuing life passions that whiz-zed by them on the runaway train of family, children, jobs.

"I'm writing about the real engine of life force, which is passion," Sheehy said. "That's why the subtitle is: 'Pursuing the Passionate Life.' "

Still, sex is right up there, in her book. And that's what her critics have focused on.

Toni Bentley, the former ballerina best known as the author of "The Surrender," an "erotic" memoir about a long, emotionally arid affair (the Washington Post Book Review called it "the apotheosis of female self-loathing"), characterized Sheehy's book in the New York Times Book Review recently as a glib attempt to deny "the mostly unavoidable humiliation that is aging — for both sexes."

"What about that intangible component called dignity? How to have it, how to keep it, how to teach it," Bentley demanded. "Because isn't life really, in the end, not so much about which passage you're in but how to behave, wherever you are?"

At this, Helen Gurley Brown, 84, rushed into the fray in Sheehy's defense, shooting off a letter published in the March 5 New York Times Book Review. She called Sheehy's book "realistic and inspiring."

"Don't mean to sound braggy," said Brown, the wife of movie producer David Brown, "but my 90-year-old playmate and I are still sexually involved — pleasurably, reasonably frequently."

Erica Jong, the author of "Fear of Flying," the best-selling 1970s novel that captured the sexual revolution from a female point of view, believes the mixed reactions to the book are rooted in American ambivalence about the sexuality of older women.

The idea that older women are as interested in sex as older men "would not be surprising in Italy or France or Europe," said Jong, whose new memoir, "Seducing the Demon," chronicles her own sensual evolution.

"There are Italian sexpot actresses who are much older. But somehow in our strangely puritanical yet sex-obsessed country, people are shocked," she said. "That's uniquely American."

Sheehy became well-acquainted with that on her book tour. At one point, she even got together for coffee with Jane Juska, the famous 70-year-old author of "A Round-Heeled Woman," to swap stories of the "curiously hostile reactions by some women to books like ours."

"For the first time in history, we have a huge population who is sound of body, agile of mind and with a maximum freedom of choice," Sheehy said. "What we can accomplish is unforeseen."

But to some women, that sounds threatening, one expert said.

The problem is that "women as they get older, they're competitive not just against their own age group, but against younger women, and against their own younger selves," said Susan Shapiro Barash, a professor of critical thinking and gender studies at Marymount Manhattan College. Barash is the author of the recently published "Tripping the Prom Queen: The Truth About Women and Rivalry."

"There is a desire to be sexual, to not be dismissed by a culture that is so age-invested and favors the young and the beautiful," Barash said. "They really do want to be just as much in the game."

"But there are women and men in the culture who question that," Barash continued. "Society tells these women that they're washed up. But the truth is, older women don't stop wanting to have sex. It doesn't go away."