Ben Stiller stars in the best American film of the year so far. I realize that may come as a shock to some people who know him primarily from the Fokker movies or those “Night at the Museum” flicks, but Stiller has always had ambitions beyond broad comedy.

Sometimes those ambitions drown him as they did in his recent directorial effort “The Secret Life of Walter Mitty.” But in the hands of the right filmmaker, Stiller can shine, as he did in Noah Baumbach’s dark comedy, “Greenberg.” And he’s even better in Baumbach’s latest, “While We’re Young,” one of the sharpest comedies in years.

Stiller and Naomi Watts play a 40-something married couple who suddenly find themselves out of step with their friends because they don’t have kids.

“I like our life as it is,” Stiller said.

“If mean if we wanted to take off to Paris tomorrow, we could,” Watts said.

“If we’re going to do it, we should plan at least a month in advance,” Stiller said.

“A month is still in the realm of spontaneity,” Watts said.

If they sound a little defensive, they are. They’re both stuck in a kind of middle-age rut. He’s been working on the same documentary film for nearly a decade and she’s still working with her dad, another filmmaker.

Stiller’s doctor doesn’t help his self-image any, either.

“You have arthritis in your knee,” the doctor said.

“Arthritis? Arthritis?” Stiller asked.

“Yes, I usually just say it once,” the doc said.

But when Stiller’s character meets an aspiring young filmmaker who professes to admire him, his life perspective is challenged in a very different way. He suddenly feels invigorated by this 20-something and his girlfriend.

“Why do you suddenly want to start hanging out with a couple of 25-year-olds?” asked Watts.

“We were just 25. I mean we weren’t. But, you know, it’ll be fun,” Stiller said.

Stiller and Watts start hanging out all the time with this young couple, expertly played by Adam Driver (of “Girls” fame) and Amanda Seyfried. They marvel at the youngsters’ exuberance for life and do their best to mimic it.

This is a rich field to mine for comedy and mine it Baumbach does. Everything the Millennials do seems like a quiet rebuke to the Generation X’ers.

“I like how engaged they are in everything,” Stiller said.

“It’s like their apartment is full of everything we once threw out, but it looks so good the way they have it,” Watts said.

Due in equal parts to their financial straits and to their hipster sensibilities, the young couple watch VHS tapes, play board games, and use typewriters. And Stiller and Watts find themselves envying that unabashedly retro life.

“While We’re Young” is a keenly observed satire of the romanticization of youth. It’s full of laughs, yes, but there’s some real bite to many of the jokes. It turns out there are serious consequences to not being true to yourself. And, in a final twist of complexity, we see even being true to yourself has its pitfalls too.