Turning the big five-oh!

Besides being three of the most famous musicians of their generation, Prince, Madonna and Michael Jackson have something else in common: They’re all turning 50 this year.

Prince’s birthday was June 7. Madonna and Jackson blow out their candles Aug. 16 and Aug. 29, respectively.

So is this cause for celebration? Or alarm?

“In two of the three cases, I’d say it’s cause for celebration,” says Jonathan Cohen, senior editor of the music trade magazine Billboard.

“As for Michael Jackson … it’s just sad. Knowing how old he is just makes it all the more creepy.”

Jackson’s career has yet to recover from allegations of child molestation (despite his acquittal in a 2005 trial), financial woes (he narrowly avoided foreclosure on his Neverland Ranch) and increasingly strange behavior that has earned him the nickname Wacko Jacko.

On the other hand, Prince is riding high after a scene-stealing performance in April at Coachella, an ultra-hip music festival in the California desert. And don’t forget His Purple Badness’ rain-soaked halftime show at last year’s Super Bowl.

Prince “has the icon status of a Paul McCartney,” says music industry analyst and former record company executive Bob Lefsetz, publisher of the Lefsetz Letter blog.

As for Madonna and Jackson, he says, “they’ve both had too many visits to the plastic surgeon. They’re trying to be young, trying to be somebody they’re not.”

On the latest Madonna album, “Hard Candy,” the Material “Girl” collaborated with several younger hitmakers, including Timbaland and Justin Timberlake.

“This record feels like a bit of a cop-out,” Cohen says. “Ten or 20 years ago, Madonna was the one discovering hungry new talents and presenting them to the world for the first time. It’s not the case anymore.”

It’s business as usual for Prince on his latest album, “Planet Earth,” released last year.

“Prince is still writing the same kinds of songs he’s been writing forever,” says Cohen. “You don’t hear him coming out with confessional acoustic ballads about middle age or things like that. I don’t think anyone really wants to hear that from him anyway.”

Jackson’s landmark 1982 “Thriller” is second only to “The Eagles: Their Greatest Hits 1971-1975” on the Recording Industry Association of America’s list of the best-selling albums of all time. But he hasn’t put out a new album since “Invincible” in 2001.

Growing old gracefully is tricky business, a contingency some pop stars didn’t count on.

“When you’re 20, you don’t even believe you’re going to be 50, never mind what you’ll be doing,” Lefsetz says.

Still, he says, “This is what they do. Wannabe musicians don’t give up when they reach 50. Why should the stars?”