Warner Bros.’ “Project X” represents one of the more successful examples in recent history of an R-rated film that directly targets an audience not allowed to purchase a ticket without a parent or guardian.

The party pic, about three high-school students who host an out-of-control rager, grossed a noteworthy $21.1 million in its opening frame at the domestic box office. Most bizzers had expected the film to land in the mid-to-high teens, with Warners holding out hope it would cross the $20 million mark.

The film’s Stateside bow puts “X” in a tidy position, inasmuch as it cost just $12 million to produce.

As expected, “X” played best with under-25 male auds; however, the film attracted a considerable female crowd, too.

Warner execs insist the rating had a nominal effect on the film’s opening perf.

“We talked about whether (theater owners) turned away audiences, and it didn’t seem to be much of an issue,” says Warner exec VP-general sales manager Jeff Goldstein, cautioning nevertheless that more restrictive films still are impacted on a market-by-market basis.

“X” will look to retain most of its under-25 audience, though Disney fanboy-favorite “John Carter” could steal away some of that young male demo with its March 9 debut.