-By Kevin Lally

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A 13-year reunion? That odd number is your first tip that something is a little off-kilter with American Reunion, the fourth movie (not counting straight-to-video knockoffs) in the raunchy comedy series that began a lot more energetically in 1999 with the hit American Pie. If this gathering of the entire original cast seems older and tired, well, that’s part and parcel with a movie whose main sentiment is that it’s tough to recapture your youthful abandon when you’re thirty-something, no matter how immature you might be.

The opening joke encapsulates what’s become of the Pie gang: The sound of squeaking bedsprings leads to a reveal of married couple Jim and Michelle Levenstein (Jason Biggs and Alyson Hannigan) bouncing in bed to soothe their young son. Now that they’re parents, the movie swiftly makes clear, their sex lives mostly consist of “private time” of the solo kind.

What of the others? Handsome jock Oz (Chris Klein) is now a sportscaster and onetime star of TV’s “Celebrity Dance-Off” and dating an uninhibited model (Katrina Bowden of “30 Rock”). Kevin (Thomas Ian Nicholas) is an architect whose free time is devoted to watching “Real Housewives” shows with his spouse. Eccentric Finch (Eddie Kaye Thomas) has taken on a sophisticated air and boasts of his world travels. And Stifler (Seann William Scott) is still Stifler: loud, uncensored, sex-crazed, and a magnet for trouble. The women from the original cast, sad to say, are barely given anything to play by writer-directors Jon Hurwitz and Hayden Schlossberg, transitioning here from the Harold & Kumar series.

The main plot drivers are Jim’s encounters with the nubile, eager, now-18-year-old next-door neighbor, Kara (Ali Cobrin), he used to babysit, and Stifler’s determination to take revenge on some obnoxious teen jocks who doused the Pie posse with spray from their jet-skis. These clashes with a carefree, sometimes arrogant younger generation only underline the melancholy at the heart of the movie; this time, the guys aren’t just sexually frustrated, they’re frustrated with themselves and what they’ve become. For series fans, the sentimental highlight should be the rekindling of romance between Oz and his old flame Heather (Mena Suvari), who’s now dating an egotistical doctor, but their scenes are so lackluster, they slow the film to a crawl.

Any hopes that Hurwitz and Schlossberg might have lent some of their Harold & Kumar zaniness to the series are quickly dashed as the film settles into predictable gags and a mood of middle-aged torpor. Reunion one-ups the first film’s memorable pie scene with some Judd Apatow-style male frontal nudity, as a sporting Jason Biggs tries to hide his goods behind a clear pot lid. But the lengthy sequence in which Biggs deals with a drunk, topless Cobrin is sometimes as uncomfortable for the audience as it is for his character.

Once again, the film is stolen by the treasurable Eugene Levy as Jim’s awkward but always supportive dad. This time, Levy gets to party with the kids, drink too much, and have a gratifying encounter with Stifler’s sultry mom, Jennifer Coolidge. Seeing these two veterans of the Christopher Guest improv comedies together again makes you forget about American Pie sequels and start to wonder: When’s the next Christopher Guest movie?