Happy birthday, Catherine Zeta-Jones. You are turning 40 this month, joining an exclusive club of women in show business who are marking the same milestone this year.

Catherine Zeta-Jones arrives at a Hollywood event earlier this year. She turns 40 on September 25.

Think big names like Renee Zellweger, Jennifer Aniston, Mariah Carey, Jennifer Lopez and Cate Blanchett.

It's a birthday many actors -- but especially female stars -- in Hollywood would once dread, hide and agonize over.

At an age where men could comfortably play heroes and lovers in the prime of their life -- and could do so for many years to come -- women often found themselves starting to be cast in different roles.

(One example: "The Graduate," where 36-year-old Anne Bancroft played a frustrated "older woman" trying to seduce Dustin Hoffman, who was just six years younger than her at the time.)

No wonder some stayed mum about their age, but this generation of 40-something female entertainers is different.

They're holding birthday bashes, embracing the big four-oh and staying as busy and famous as ever. Their faces grace magazine covers, their bodies are the envy of women decades younger and their careers seem to be going strong.

Zellweger herself has said she hasn't noticed good parts getting scarce.

"I don't see it. I don't know. I'm so spoiled with respect to the experiences I've had and the opportunities I've had that I don't see it," she said, according to published news reports.

So has anything changed for women hitting 40 in a business obsessed with youth?

Industry observers say stars like Zeta-Jones Zellweger and Aniston have more options than ever to have long, distinguished careers thanks to independent films and television, but opinions are mixed on whether they can sustain a big Hollywood presence.

"These women are still among the most bankable, biggest stars in the industry and turning 40 isn't going to change that," said Kathy Heintzelman, entertainment director for More, a magazine geared toward women over 40.

Importance of box office

But others argue that the opportunity to land lead roles in major studio films for actresses like Aniston, Zeta-Jones and Zellweger is running out.

It's all about the global box office for Hollywood, which right now is most interested in making comic book movies or big-action, special-effect films where women are mostly decorative, said Leah Rozen, film critic for People magazine. That often leaves few lead roles in big-budget Hollywood films for established female stars as they get older.

"They're going to work, but the entire movie will not be built around them. It will not be a Renee Zellweger vehicle or a Catherine Zeta-Jones vehicle, unless it's an indie film," Rozen said.

"I'm not saying anything about the quality of the movies they're making or their validity as actresses," Rozen said. "I am saying that viewed as box office -- or can they open a movie? -- the answer would be no right now."

Zellweger's most recent project, "My One and Only," which Rozen called a "very sweet little film," has grossed less than $1 million since it premiered last month (it opened in wide release last Friday.) Earlier this year, her film "New in Town" grossed about $16 million domestically, according to Boxofficemojo.com.

Zeta-Jones' last big Hollywood role was in the 2007 movie "No Reservations," which earned $43 million in the U.S.

Aniston has had a better run. Last year's "Marley & Me" took in $143 million domestically and more recently, "He's Just Not That Into You" earned $93 million.

Men are viable as box office leads far longer than most women, Rozen said. Think Harrison Ford, who was still the action hero at 66 in last year's "Indiana Jones and the Kingdom of the Crystal Skull" (which earned $317 million domestically) or Sean Connery, who at 69 was paired with a then-30-year-old Zeta-Jones in 1999's "Entrapment" ($87 million domestic gross).

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