Noah Baumbach’s brisk new comedy “While We’re Young” is very funny and sublimely morbid about topics invisible to modern movie humor. It flits from characters hitting mid-40s middle age to the addictive nature of artisanal ice cream, then zips between intergenerational frenemy relationships to the nostalgic impact of vintage music on lo-fi vinyl.

It’s a satire of endless youth, “The Kids Are All Right” colliding with “My Generation.” Yet it’s the best kind of grown-up absurdism. The actors play every laugh line straight, like Buster Keaton at his most deadpan, because that’s what makes them funny. If you haven’t been deeply in love with Baumbach’s work since his sadly sweet 2005 gem “The Squid and the Whale,” get ready. He’s back.

When we meet New Yorkers Josh and Cornelia (Ben Stiller and Naomi Watts in peerless form) their relationship is getting moth-eaten. She’s the producer for her father, Leslie (Charles Grodin), a superstar documentary filmmaker. Josh, who met and married her on assignment for one of the great man’s shoots, has been trying to equal Leslie’s showpiece nonfiction with his own stale efforts.

Cornelia feels outlapped by her new-parent best friend. Josh is perpetually puttering with his unfinished 6½-hour project about that ever-exciting topic, Turkish politics.

Baumbach is fascinated by such tense neurotics; remember the cynical indie-band dud of “Greenberg” and the longing also-ran female dancer in “Frances Ha”? His new characters, entering their 40s without children or exciting communication, miss the romantic younger versions of themselves. Then they meet a married pair of 25-ish millennials who make their Gen X 20s seem prehistoric. Cue irksome upheaval.

Jamie and Darby (Adam Driver and Amanda Seyfried) aren’t just cool, they are tragically hip, kind of like the older couple but deeply different. Jamie, a docu lover, raves about Josh’s only finished film with the enthusiasm of a huge fan. He has nonfiction aims, too, and loves to ask Josh about the road map of his career. Josh relies on smartphone Google to answer all of life’s questions; Jamie simply shrugs “Let’s just not know.” Darby makes organic non-dairy gelato, and blows the lid off at her hip-hop dancing class, which makes Cornelia join, though she does more bunny hop than boogie down.

They invite the oldsters to hang out at their shoddy industrial loft, filled with books in print, board games and audio cassette tapes. It looks like one of those retro-gizmo palaces you see in a film from Baumbach’s frequent partner Wes Anderson, but slightly more realistic. Amazed at the duo living surrounded by “all the things we threw away,” Josh sees them as irresistible friendship recruits, freeing him at a time when the only emotions he still felt were “wistful and disdainful.”

Soon the quartet have the montage-worthy outings you see in jewelry store romance videos — lunch dates, street parties. Josh and Jamie go cruising Brooklyn on bikes, set to 2Pac’s thumping “Hit Em Up.” When a freak injury sends arthritis-prone Josh to the doctor, things start taking a jarring turn. Josh begins to see Jamie’s interest as just a sneaky step to building his own film career, pursuing attention more than art. Cornelia questions her husband’s goals and hates that punk fedora he’s begun wearing. The odd couples don’t fit together either as a foursome or a pair of pairs.

Stiller and Watts are wonderful as the dysfunctional couple, a smart man and woman acting out like clever children chasing a bohemian fantasy of adult life. “While We’re Young” is fast, urbane wit, right down to the minor parts. The Beastie Boys’ Adam “Ad-Rock” Horowitz is great in the role of Josh’s increasingly square old pal, and Matthew Maher is long-suffering perfection as Josh’s unpaid film editor. Everyone fits Baumbach’s regular goal of being critical yet sympathetic. He has a fine touch for watching his semi-heroes with fondness even when they’re acting ridiculous. Especially when they’re ridiculous.