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Question: Did you hear the one about Helen Mirren’s no-nudity clause?

Answer: Yeah, it kicks in on her 75th birthday.

Yuk, yuk, yuk.

Conventional wisdom has it that an actress gets more leverage the older and more established she becomes. Conventional wisdom also has it that the consumer demand for on-screen female flesh tends to gravitate toward actresses under 30. Heck, under 25.

How, then, to account for the recent partial or complete nude turns of actresses such as Mirren and Julie Walters (both in the just-released “Calendar Girls”), Diane Keaton (“Something’s Gotta Give”) and Kathy Bates (2002’s “About Schmidt”)?

In case you’re wondering, Mirren is 58, Walters will be 54 in February, Keaton hits 58 in January and Bates is 55. None of them have ever been considered traditional sex symbols, and all except Keaton have done on-screen nudity before. Keaton, in fact, gained a certain notoriety for refusing to go the full monty in the stage version of “Hair” in the late 1960s.

Not this time.

“I had to do it to be honorable to the movie,” Keaton told the Daily News. “Look, the film is about middle-age love, and this is a woman’s body naked, and this is a man’s bottom just flailing about. Jack’s [Nicholson] making fun of himself. Why shouldn’t I make fun of myself?”

Slightly less daring — though no less undraped — are the recent disrobings of Meg Ryan, (age 42, in “In the Cut”), Emma Thompson (44, HBO’s “Angels in America”), Mary Louise Parker (39, “Angels in America”) and Laura Linney (39, “Love Actually”).

Angelenos saw a nude Jerry Hall (47) in the stage version of “The Graduate.” Other actresses to bare it all as randy Mrs. Robinson include Lorraine Bracco (54) and Kathleen Turner (who was 48 when she originated the role.)

Mature subjects

If the display of slightly older flesh is to be looked on as a trend, pop-culture critics and filmmakers say it dovetails with a cinematic tendency toward stories about men and women of a slightly older age. There may be a narrative place for Amanda Peet and Keanu Reeves in “Something’s Gotta Give,” but the bulk of the film’s best jokes concern stamina, Viagra, menopause, fading eyesight and, yes, flesh somewhat past its prime. Keaton may go nude, but a much bigger joke is made of the exposed behind of her 66-year-old co-star, Nicholson.

Harry Sanborn, Nicholson’s rakish music mogul, has never dated a woman over 30 and he’s supposed to be seriously freaked out when he accidentally discovers Keaton’s Erica Barry in the buff.

“And Jack Nicholson is no poster boy,” says Elayne Rapping, professor of woman’s studies and media studies at the State University of New York at Buffalo. “The message you’re getting is that older women’s bodies are really not very sexy. That is a real problem in American culture that you don’t find in European culture.”

Apologists would argue that it is shock, not horror, that floored Harry. Erica is, even at 57, a knockout who ends up besotting both Harry and Reeves’ Dr. Julian Mercer.

Not so Kathy Bates, who came on to Jack Nicholson’s character during a much-discussed hot-tub scene in last year’s dark comedy “About Schmidt.” Playing characters who are frequently plain and/or overweight for the bulk of her career, the Oscar-winning Bates (“Misery”) has done nudity both onscreen (“At Play in the Fields of the Lord”) and onstage (“Frankie and Johnny in the Clair de Lune.”). “About Schmidt” earned her a third Oscar nomination.

“You do that once, and it’s interesting, maybe shocking in the right way,” University of Colorado professor of English Anne Kibby says of the hot-tub scene. “To make that a general trend, you’re avoiding the issue of why Hollywood has such a difficult time writing parts for older women. I think the audience is there, but the scripts aren’t.”

One person inclined to agree is William H. Macy, who, at age 52, shot the first nude love scenes of his career with 35-year-old Maria Bello for “The Cooler.” Needing some comfort from “an old friend: J&B” to get through the steamy scenes, Macy believed they were an important part of “The Cooler’s” love story. What’s more, he says, adult relationships can be just as hot as anything the MTV generation cooks up.

“There are a lot of us Baby Boomers out there. We’re still in really good shape, and we’re horn dogs like we’ve always been,” says Macy. “And there are some fabulous-looking women in their 40s and 50s. I’d like to do a romantic comedy and do some sexy scenes with them. I think there’s an audience for that.”

Better with age

As for the “Calendar Girls” women, well, the very point of that movie — summed up poetically by one of the few male characters — is that the women of Yorkshire become better looking the older they get. Hence, the scheme hatched by Chris Harper (Mirren) for local members of the Women’s Institute to shoot and publish a calendar of WI members performing WI activities in the nude is anything but a daft notion.

The film, directed by Nigel Cole (“Saving Grace”), is already the fifth-most-successful film of all time in Britain. Occupying the top spot — and speaking of less-than-pristine nudity — is “The Full Monty.”

The actual calendar, published by the Rylstone chapter of the WI, has sold more than 300,000 copies and raised more than $1 million for leukemia research. That’s a ringing endorsement for the cause, says Walters, but also for the images themselves.

Redefining beauty

“I think the movie puts out a great message that beauty is not just about youth,” says Walters, who plays Annie, Chris’ best friend. “Hollywood has always been about the screen goddess: women who are big, beautiful, perfect and unreal. That’s sort of continued down, and here are these real women without face-lifts and everything else.

“But as you go through life, you exchange that youth for something else, hopefully for some kind of wisdom. Or at least for the fact that you’ve lived a life and the beauty’s in who you are in the life that you’ve lived.”

Performing the scenes

A lofty sentiment, that. So with that principle firmly in mind, actually performing the nude scenes must have been a cakewalk, right?

Right …?

“I was embarrassed that day,” confesses Walters. “I was glad when it was over. I’m still embarrassed. It was interesting. When the photos were all developed and we saw them, each of us privately hated our own and thought everyone else’s was wonderful.”

Making the task easier, says Walters, was the cheerleading presence of Mirren (“Gosford Park,” “Prime Suspect”), long regarded as one of Britain’s least-shy actors where nudity is concerned.

Mirren, whose last film before “Calendar Girls” was the steamy “Roman Spring of Mrs. Stone” for Showtime, says that disrobing has actually become easier later in her career.

It’s much harder when you’re younger and there’s a sort of sexual objectivity,” says Mirren. “It’s sort of liberating now. I don’t understand why people make such a fuss over it.”

Her director can think of a reason.

“Helen Mirren is still one of the sexiest women on the planet. I don’t care if she’s 58 years old,” says Cole. “She’s gorgeous — and you get to see her naked. That’s got to be worth something.”