Noah Baumbach has always had a dash of hypochondria, but in the last few years, his doctor's visits have changed. “If you're a worrier like I am, or Ben,” he says, referring to Ben Stiller, the star of his 2010 movie Greenberg, “you're used to going, 'Is this something that should be cause for worry?'” Before, the doctors would smile patiently and say no. Now that Baumbach and Stiller are 45 and 49, respectively, the doctors order an MRI. Baumbach jokes, “Shit — now you're taking me seriously.”

While We're Young, the title of Baumbach and Stiller's new film, is a mild slap to the face of worriers in their 30s and 40s who, having aged out of being prodigies, prematurely fear they're old and irrelevant. Middle age is exactly that: You still have half your life ahead of you. But it's hard to give up being precocious. “You're used to having that reaction, 'Oh, you're just a baby,'” says Baumbach, who made his first film, Kicking and Screaming, at 26. “And then people stop saying that.”

Stiller, too, started young. At 24, he was writing for Saturday Night Live. Soon after, he launched The Ben Stiller Show and then wrote, directed and starred in 1994's Reality Bites, branding him the face of Generation X, a label Baumbach tried to fight off when Kicking and Screaming came out the following year. “I hated the idea then,” Baumbach confesses. “I was just trying to make this movie about me and my friends.”

But today, both men — and the generation they reluctantly represent — must pass the torch to the millennials while hovering close to their light. In the last three years, Baumbach has made two films about 20-something angst, Frances Ha and the upcoming Mistress America, both starring his 31-year-old girlfriend, Greta Gerwig, though her status as a millennial is up for debate. “Is my 5-year-old son a millennial?” Baumbach muses, imagining out loud if his boy went full Brooklyn and churned his own butter.

While We're Young wrestles with aging and maturity, which aren't always synonymous. Stiller plays a dawdling documentary filmmaker who made one good film a decade ago and has been anxiously tinkering with his second ever since. Childless in their early 40s, Josh and his wife, Cornelia (Naomi Watts), are suddenly hipsters without a tribe. Their closest friends (Maria Dizzia and Beastie Boys' Adam Horovitz) are absorbed by a new baby, and drunken group weekends at the lake house have turned into diapers and rattles with a bunch of squares they can't stand. Where do you belong when your peers have grown past you?

“Options you imagined are closed off,” Baumbach says. “It can be liberating, but it's painful. Everybody has to come into some kind of contact with the person that they're actually being instead of the person they thought they hypothetically might be.”

But Josh and Cornelia's stasis is a different kind of panic than when the baby boom generation hit 40 and felt trapped by past decisions. In response, their children have been afraid to make any decisions at all. Adulthood isn't a must; it's a choice. “My parents' generation, this was much rarer,” Baumbach says. “They'd just all be in second marriages.”

When Josh and Cornelia meet an early-20s married couple, Jamie and Darby (Adam Driver and Amanda Seyfried), the pair is re-energized. Forget boring brunches — now they're tripping on ayahuasca. Ten years ago, the joke would be Jamie and Darby teaching their elders how to text. But to Josh and Cornelia's surprise, today's cool kids are technophobes who elbow them to put down their smartphones. The dream of the '90s is alive in its toddlers.

Josh fancies himself a mentor to Jamie, an aspiring director himself. It's a dynamic Baumbach gets, as he was befriended by Brian De Palma, Peter Bogdanovich and Mike Nichols. (“I'm aware of what I imagined these people would be versus who people actually are.”) There's a touch of All About Eve in this fictional friendship, especially when Jamie's new documentary is so hip that he's guaranteed to be cooler than Josh ever was. Meanwhile, Josh's overly ambitious follow-up has dragged on so long that people no longer care to hear what he has to say. Baumbach admits, “Even the best version of that movie is still going to be No. 2 on the marquee at Cinema Village.”

Baumbach is less afraid of teamwork. He and Gerwig co-wrote the script for Frances Ha, and here, several lines of Darby's dialogue come straight from Gerwig just being herself. “She's very quotable,” Baumbach beams. “People my age grew up with this idea of genius and the lone artist who creates in his room. Obviously, now, there's a more group-collaborative idea and appropriation.”

Onscreen, Baumbach has immortalized his whole life to date, from his childhood in The Squid and the Whale to the questions he's asking himself today. “They don't feel like home movies,” he explains. He can't rewatch them. “It feels like some other version of myself who made it.”

Instead, they're like notches on a doorframe: a timeline of growth. “I chart my life by movies,” Baumbach says. “When people say, 'What year was that?' I can remember only by what movie I was making.” That's as good a reason to keep on creating as any.

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