When you’re young, to be old — even just the 44 kind of old — is unimaginable. Yet no one who has ever turned 44, or 54 or 74, can tell you how it happened. There’s no single moment of passing to the other side of the looking glass; the only thing that’s real is the bewilderment of realizing that you’ve somehow squeezed through it.

That bewilderment is the guiding force of Noah Baumbach’s fearless half-a-comedy While We’re Young, an unsparing consideration of what makes the young different from the not-so-young. Baumbach’s eighth feature isn’t just sharp, it’s serrated; its jokes — and there are lots of them — come at you with rows and rows of tiny teeth. But even if Baumbach, who also wrote the film, betrays annoyance with the sense of entitlement and soufflé-high overconfidence of millennials, in the end he comes down hardest on his beleaguered semi-hero, played by Ben Stiller. He’s the one we most frequently laugh at, but also the one we feel the most for. Youth may be wasted on the young, but midlife ennui is unbecoming, and While We’re Young refuses to give in to middle-aged self-pity.

Stiller plays a onetime documentary filmmaker who’s hit his forties and stalled out on the masterpiece he’s been painstakingly crafting for years, a windy chronicle about society, power, and the post-industrial military complex (or something like that). We don’t even have to see the stultifying clips from Josh’s would-be magnum opus to know what the problem is: Even his name, Josh Shrebnick, is like a head-thwap from a Don Martin cartoon. Josh’s wife, Cornelia (Naomi Watts), produces documentaries — just not his. Instead, she works with her father, lauded documentarian Leslie Breitbart (Charles Grodin), whom Josh respects and resents in equal measure, for understandable reasons: Breitbart is like the Maysleses, Pennebaker and Wiseman rolled into one, and Josh knows he’ll never be as good.

But he gets a jolt when he meets young aspiring filmmaker Jamie (Adam Driver) and his artisanal-ice-cream-maker wife, Darby (Amanda Seyfried), both about twenty years his junior, who fawn over him shamelessly. He’s inspired by their energy, their casual generosity, their drive to “make stuff,” and while Cornelia isn’t sold at first, eventually she, too, falls prey to the couple’s charms.

It turns out that Jamie is a climber of the worst sort and, tragically for Josh, perhaps one with actual talent. We cotton to Jamie’s selfish ambitions before Josh does, in the way Jamie and Darby mutter a distracted “thanks” when Josh reaches for the dinner check, in the way Jamie gushes over the single documentary that earned Josh some mild fame long ago, in the way, upon meeting the living legend Breitbart, Jamie clasps his hands in an ingratiating, phony-baloney pranamasana.

The point isn’t that all young people are ambitious, careerist creeps. But Jamie, whom Driver plays with a kind of maniacally shambling charisma, uses the wide-eyed faux openness of his generation as a smokescreen for his ruthlessness. No wonder Josh, in his misguided, self-conscious earnestness, falls for it. Even Grodin’s crotchety old Breitbart says of Josh, in one of the film’s most piercing moments, “He wants what I have, but he’s not merciless enough to get it.” And by the time Josh has all the evidence he needs to expose his young friend’s careerist duplicity, it’s too late — and it seems no one really cares, anyway. In the movie’s pivotal scene, Josh stands up for everything he believes in, only to be met with a rousing “So what?” This is the painful centerpiece of Stiller’s performance: For much of the movie, he struts around like a thundercloud with eyebrows attached, but in this instant, the stillness of his isolation folds around him like a set of invisible wings. It’s such a bristly, complicated moment, and such a despairing one, that you wonder how he and Baumbach are going to reel the movie back from the brink.

They do it, of course, with a laugh, though nearly everything that’s funny in While We’re Young comes wrapped in barbed wire.

Baumbach made his debut twenty years ago with Kicking and Screaming, about a group of recent college graduates who couldn’t move away, either physically or spiritually, from the cozy, cloistered world they’d built for themselves over the past four years, a world where they could crown themselves imaginary kings. If the characters were aimless, the movie around them wasn’t: Even its wistfulness had jagged corners. In the years between, Baumbach has made some fine pictures (Frances Ha) and some deadening, hermetic ones (Margot at the Wedding), but it’s While We’re Young that really fulfills the promise of his brash but fine-grained debut. These new characters, like fast-forward versions of the first ones he wrote, have woken up middle-aged without a clue as to how they got there. They all but say, “We knew we’d be different, but we didn’t think we’d be this.”

While We’re Young embraces the this, facing the bittersweet truth that it’s all we’ve got. Better yet, it floats the possibility that the people we thought we’d be were overrated.