‘While We’re Young’ takes aim at the old and the new

"While We're Young" is a diamond-hard comedy about the worst time in a person's life: that moment when he (or she) looks in the mirror and no longer sees the hip young thing he (or she) still thinks is there.

It happens to all of us, and now it's Generation X's turn; it couldn't happen to a nicer or more deserving group of people. (That cruel laughter you hear is from all the gray-haired baby boomers in the audience.) In movies like "The Squid and the Whale" (2005) and "Greenberg" (2010), writer-director Noah Baumbach has proved a master at dissecting boho neuroses, and he further ups his game here, delivering a chatty, observant social farce that manages to be both sympathetic and utterly without pity.

For most of its running time, at least. Ben Stiller plays Josh, a struggling Manhattan documentary filmmaker, as if the character were Roger Greenberg's kinder, gentler, more socialized brother. He's a basket case but a functioning one, with a loving wife, Cornelia (Naomi Watts) and — real estate being identity in New York — a cozy, book-crammed apartment. Despite a film project that has consumed years of his life and is so intellectually high-pitched that only dogs and academics can understand it, Josh still believes, in his 40s, that he's relevant. The sap.

Yet he falls head over heels in love with a young couple who show up at one of his lectures, and so does Cornelia. Jamie (Adam Driver) and Darby (Amanda Seyfried) are in their 20s and blissfully, unselfconsciously hep. Baumbach has fun depicting their casual disdain for anything manufactured or new. A brilliant little montage shows the older couple using the technology — laptops, iPods, cellphone videos — they feel is their birthright while Jamie and Darby listen to vinyl records, write on manual typewriters, watch VHS tapes. Darby has a sideline in artisanal sorbet (this week's flavor, avocado and almond milk). When Josh can't remember the word "marzipan" and pulls out his
iPhone to look it up, Jamie says "Let's just not know what it is." Who wouldn't want to be like these people?

So Josh is soon wearing flea-market wingtips and hipster fedoras, and Cornelia has signed up for hip-hop exercise classes, and they're both riding bikes. Their friends, the ones their own age, are aghast. Maria Dizzia (the heroine's friend in "Orange Is the New Black") and Adam Horovitz (Beastie Boy Ad-Rock, all growed up) are grumpy and marvelous as new parents who can't fathom why their pals have started behaving like Gen-X mutton dressed as Millennial lambs.

Baumbach has something of an evil genius for casting. If Driver — the mercurial Adam of "Girls" — and Seyfried are solid as the incoming kids, Charles Grodin (the original "Heartbreak Kid") ruthlessly represents the boomers refusing to cede the stage. He plays Cornelia's father, a legendary documentarian in the Frederick Wiseman mold.

And who's that fubsy professor at the center of Josh's own never-ending film project? It's Peter Yarrow, minus his one-time singing partners Paul and Mary. Like Luke and Leia in the trash compactor, Josh and Cornelia are in danger of being crushed by generational cool on both sides.

Jamie's a filmmaker as well, at ease with the technical and ethical fluidities of social media, and he gradually sucks Josh into a documentary project involving Facebook friends and their various real lives. At a certain point, about two-thirds of the way in, "While We're Young" changes tone and direction; it becomes a more plot-driven movie and a lesser one, something along the lines of "All About Eve Goes to Williamsburg." It's still entertaining but with an edge of resentment, even finger wagging, that curdles the acerbic fun.

Baumbach, it turns out, may have his own blind spots. Is it just me, or does Josh's guilty attraction to the wide-eyed Darby carry echoes of the director's own collapsed marriage to actress Jennifer Jason Leigh and subsequent infatuation, real-life and cinematic, with then-20-something Greta Gerwig, star of his "Frances Ha" (2012) and upcoming "Mistress America"? Maybe that's irrelevant and maybe it ain't; don't forget that Baumbach's the filmmaker who repurposed his parents' divorce into "The Squid and the Whale." Or that he did it extremely well.

More to the point, "While We're Young" devolves, just a little, into a lecture about Kids These Days, one only partly softened by the director's wit and the fact that the lawn he's telling them to get off is New York City. Baumbach may be the sharpest social satirist of his generation — in some ways, he's a Woody Allen with clearer vision and fewer nervous tics — but the message of this droll, enjoyable bourgeois vaudeville can be boiled down to Father Knows Best.