Barry Manilow may be getting old — but his fans are getting even older.

That’s the whisper on Broadway this week, as the 76-year-old “Copacabana” crooner is set to face an ocean of empty seats at a 17-show stint that’s slated to open Friday, sources told The Post.

The Brooklyn-born singer-songwriter — who, even during the 1970s, gained a reputation for attracting throngs of women a generation older than he was — has sold barely half the seats for the upcoming run at the Lunt-Fontanne Theatre.

Opening night and Saturday’s show are nearly sold out, with so-called “Fanilows” lining up to hear a string of decades-old hits like “Could It Be Magic,” “Can’t Smile Without You” and “Looks Like We Made It.”

Nevertheless, about half the seats for the next three weeks — priced in the $100 to $300 range — are still empty despite the fact that the Lunt-Fontanne only has a capacity of 1,500.

That’s in stark contrast to the recent success of a few of his slightly younger peers. In December, 69-year-old Bruce Springsteen ended a 15-month Broadway residency that boasted 236 straight sold-out shows. Meanwhile, 70-year-old Billy Joel has been selling out a monthly show at Madison Square Garden since January 2014.

Manilow’s looming flameout has surprised a few insiders, who note that in April he extended a residency in Las Vegas after performing his 500th show at the International Theater at Westgate.

“The Vegas crowd is all tourists who will buy tickets or discount tickets to just about anyone who’s playing — even if the performer, like Barry, hasn’t had a hit in years,” one insider said. “The Broadway crowd is more sophisticated and a tougher sell.”

When they announced the upcoming three-week run in May, promoters, including Live Nation, cited Manilow’s long history on Broadway. He won a Tony Award for his 1977 debut and boasted sold-out, eight-week runs at the Gershwin Theatre and St. James Theatre in 1989 and 2013, respectively.

Neither Manilow’s manager and husband, Garry Kief, nor his publicist, Victoria Varela, returned calls for comment.