Filmgoing by the under-25 crowd is declining — and for a multitude of reasons.

B.O. observers say it’s because young moviegoers think ticket prices are too expensive, that they have too many alternate ways of seeing a film, and they’re not interested in 3D and non-franchise pics. Surveys bear out those assertions.

Under-25 auds haven’t been supporting youth-targeted titles on opening weekend of late. But more upsetting to the film biz is the fact that they’re not getting around to seeing films in theaters in subsequent weeks,

either, resulting in a steady decline over the past five years of multiples (the figure derived by dividing a pic’s final box office tally by its opening-weekend gross).

“The avenues to spread word of mouth, especially among younger moviegoers, have increased exponentially,” said Vincent Bruzzese, prexy of the Worldwide Motion Picture Group at the Ipsos OTX research/consultancy shop. “But the multiples are declining.”

Around this time last year, “Jackass 3D” had cumed $101 million after just three weeks — two times its $50 million opening. By comparison, “Paranormal Activity 3,” which debuted to $52 million the weekend of Oct. 21, has cumed $95 million during the same play period as “Jackass.”

Is it a cyclical trend due to the films in the marketplace or a permanent shift in moviegoing behavior?

Fox senior VP of domestic distribution Chris Aronson attributes some of the decline to higher ticket prices. “We, as an industry, have made it difficult for the casual moviegoer,” Aronson said. “When times were tough, they went to the movies, but that’s not necessarily the case anymore.

“That’s seems particularly true for teen audiences who have less disposable income,” Aronson said.

The average multiple for teen-targeted films has dipped consistently over the past five years, from 3.63 in 2007 to an average of 2.47 this year.

According to a survey conducted by Ipsos’ OTX Motion Picture Group, 55% of under-25 filmgoers polled this year said movies are an affordable form of entertainment compared to other activities, down from 63% in 2010. That’s partly caused by a shift in the perception of 3D pics, the survey indicated, as well as higher 3D ticket prices. (The survey, taken in August both years, polled some 2,500 moviegoers under 25.)

In 2010, 43% of under-25 auds said 3D was a better presentation than 2D, but that figure decreased this year to 37%.

The decline in 3D popularity didn’t affect this past weekend’s 3D share for Warner Bros.’ “A Very Harold and Kumar 3D Christmas,” which earned 95% of its opening from the format. But the film had 88% of its total locations in 3D.

Higher 3D ticket prices weren’t able to make up for the decline in under-25 attendance. “Harold and Kumar” bowed with $13 million, considerably lower than industry expectations.

Unsurprisingly the season’s top-performing pic among under-25s is not in 3D.

“Paranormal 3” is the only post-summer release to work with the younger demo, scoring the best-ever opening in October with $52.6 million. But even that film, which benefited from a loyal fanbase, drew nearly half of its opening (at 47%) from auds over 25. Last year, “Paranormal 2” earned 40% of its opening from over-25s.

In 2008, Paramount-DreamWorks’ late-September release “Eagle Eye” displayed considerable gusto with under-25 auds, opening to $29 million and cuming nearly $100 million through early November, while 2009’s $32 million opener “Zombieland,” from Sony, cumed almost $75 million during a comparable sesh.

This year’s nonsequels, such as “In Time” and “Footloose,” are having a tougher time in theaters. Fox sci-fier “In Time” debuted last weekend to a soft $12 million. Though auds were familiar with Par’s “Footloose” property, the Oct. 14 opening was still softer than expected for the musical remake at $15 million.

“Moviegoers have to take some responsibility too,” Bruzzese said. “They claim to want more original content, but they’re not going to see it.”

B.O. pundits insist that movies have to compete with more forms of entertainment than ever before, with the economy limiting consumers’ amount of expendable cash. A choice of digital platforms is affecting how younger moviegoers especially are viewing movies.

According to the OTX study, 48% of males under 25 have less leisure time than in 2010. Meanwhile, fewer young women are seeing films in theaters, while more are watching DVDs and VOD at home. The largest change in behavior involves watching streaming entertainment services such as Netflix, with a jump of 14% over the past year.

While this trend has impacted many releases, the penultimate “Twilight” offering will be an exception.

Bruzzese said films appealing to groups of teens perform best. “They’re going for that social experience,” he said.

“The Twilight Saga: Breaking Dawn — Part 1” should create a teen frenzy when it launches in two weeks, though it’s less certain whether that crowd will continue to visit the plexes when the frenzy has died down.