Written and directed by Mike Mills, based on the novel by Walter Kirn

Starring Lou Pucci, Tilda Swinton, Vince Vaughn, Vincent D’Onofrio, Keanu Reeves, Kelli Garner, Benjamin Bratt, Chase Offerle

Rated R; 96 minutes

A mature film about immature people of all ages, Thumbsucker is one of the better troubled-teen-coming-of-age stories to be seen in recent years, in large part due to a dream cast of independent film stalwarts and a revelatory performance by Lou Pucci in the title role.

Pucci plays Justin Cobb, a 17-year-old who still finds comfort in sucking on his opposable digit at times of stress. He and his family want him to stop, but they’re not really much help — they each have their own addictions, which are not as visible as Justin’s but are potentially just as harmful. His father Mike (Vincent D’Onofrio), sidelined from a professional football career by an injury 20 years ago, can’t deal with the reality of his own aging — he insists Justin call him by his first name because “Mike says when I call him ‘dad’ he feels old.”

Mother Tilda Swinton hides her neuroses more effectively than either her husband or her son, but she has her own obsessions: her “thumb” is her infatuation with a coked-up TV actor (Benjamin Bratt) that she gets to meet in person when she takes a job in a fancy rehab center. Vince Vaughn, in a funny turn as Justin’s debating team coach, tries to mentor Justin, but he’s caught between his job as an authority figure and his palpable envy of his students’ youth. Vaughn captures the paradox many teachers find themselves in: they have to be the adults in a sea of hormonally charged adolescents, so they simultaneously feel old as the hills and younger than springtime.

These are just a few of the people, ranging from well-meaning to self-important to self-deluded, who offer Justin advice, hypnosis, medication (both legal and illegal) and sex throughout the film, but as Linus from the “Peanuts” cartoon strip proved, there’s really nothing that can replace a thumb.

This description makes Thumbsucker sound like an absurdist black comedy that John Waters might have made, or a chilling satire on our get-fixed-quick culture à la Todd Solondz ( Storytelling) or Todd Haynes ( Far From Heaven). While Thumbsucker has some elements of both genres, writer-director Mike Mills successfully steers a more difficult course, where the film’s characters are recognizably human and often deeply flawed, without their ballooning into grotesques and caricatures that would allow us to laugh them off. (Alexander Payne’s Sideways walked a similar tightrope.)

Not that the film doesn’t have humor, much of which comes from Keanu Reeves (I was surprised too). As Justin’s orthodontist/hypnotist/guru, Reeves slyly mocks his Matrix-spawned mantle of world-saving seriousness while still portraying a character who disguises his raging narcissism with dollops of semi-mystical “wisdom.” But like the stopped clock that’s right twice a day, Reeves stumbles on a truth, delivering what may be the film’s message, albeit qualified: “The trick is living without an answer…I think.”

In addition to eliciting a strong performance from Reeves, Mills provides sure-footed direction to the entire cast. Pucci, who has already pocketed some acting awards for this role, makes each of his transformations (from self-doubting thumbsucker to Ritalin-powered debating team ubermensch to a potheaded sex object for classmate Kelli Garner) consistent and believable — no mean feat considering Mills puts him through a whole catalog of teen traumas and “cures” throughout the film. Pucci makes a passive character interesting, a trick that can hamstring actors twice his age. His delicate, almost feminine face can be remarkably expressive, and if he can work on his vocal technique a bit he will be an actor to reckon with.

As the mother, Swinton brings depth to what could have been either a cartoony bitch or a too-perfect mom. Like Joan Allen’s great work in The Upside of Anger, Swinton lets us see the multiple roles that mothers play in our lives — flirty friend, fierce protectress, a waif scared of being left behind — though Swinton gets more support from her writer/director than Allen did from Mike Binder.

D’Onofrio also proves himself the master of the awkward physical moment, playing an inarticulate guy who can still be surprised by his own feelings. Mills, who has directed short films, music videos and commercials, makes a strong showing in his feature film debut by balancing all these very different actors’ performances into a thoughtful, funny, moving film.