Aging is an inevitable fact of life, an unavoidable process that slowly but surely changes us physically, if not always emotionally. Thirty- and forty-somethings may no longer experience the feelings of invincibility and idealism they had in their twenties, and they may be surprised every day by the new aches and pains of their bodies, or the lines on their faces when they look at themselves in photographs. They may go to bed earlier than they once did, and their days of late-night parties and frivolity prove fewer and farther between as adult responsibilities take over. Even if it seems as if no time has passed at all, a person can blink and twenty years have passed, the man or woman looking back at them from the mirror no longer matching up to how they feel on the inside. These existential thoughts were clearly on 45-year-old filmmaker Noah Baumbach's (2013's "Frances Ha") mind when he sat down to write "While We're Young," and they shine through with resounding truthfulness and humor.

44-year-old college professor/struggling documentarian Josh Srebnick (Ben Stiller) and his wife, 43-year-old part-time producer Cornelia (Naomi Watts), live what they believe is a happy, comfortable life together in Brooklyn. Granted, not all of their personal and professional aspirations have been met, but when they fail to click with the newborn baby of longtime pals Fletcher (Adam Horovitz) and Marina (Maria Dizzia), they begin to wonder if it wasn't fate that they were never able to become parents. When two of Josh's classmates, married 25-year-olds Jamie (Adam Driver) and Darby (Amanda Seyfried), invite him and Cornelia out to eat, it begins an unlikely friendship between the foursome. Josh likes the feeling of being admired by Jamie, who claims to be a fan of his little-seen last documentary, but he is also intoxicated by their lax, hopeful attitude and virtually care-free spontaneity. He and Cornelia do not realize how much they've changed until they start hanging out with the much-younger Jamie and Darby. They might want to go back to the way they were when they, too, were in their mid-twenties, but the hard reality is much different than how they romanticize it in their minds.

"While We're Young" isn't about emotionally stunted adults as much as it is about the universal desire to recapture a time both fleeting and past, and the ultimate acceptance that comes with embarking on the next natural phase in one's life. As much as Josh and Cornelia try, they can never go back. Not fully. And as Josh finally realizes, "There are things I'll never do, and things I'll never have." The film, honest yet unexpected, diverges from the usual tropes about the disentanglement between two separate age groups. As Baumbach succinctly sees it, it is the Gen-Xers who have fully embraced a tech-savvy lifestyle of multiple mobile devices, computers and Netflix, while it is the hipsters—the kind who wear hats, collect vinyl rather than CDs, and watch "The Howling" on VHS—who see anything retro as being cool. A nice little detail is Jamie and Darby's music preferences, they and their same-aged friends listening without irony to Lionel Richie's "All Night Long," Survivor's "Eye of the Tiger," and Billy Ocean's "Get Out of My Dreams, Get Into My Car" because they weren't even born when they came out and have discovered them with fresh ears. As self-involved and flippant as these two can be in certain respects, there are also positive and unique attributes that keep them from becoming caricatures. Baumbach isn't interested in generalizing, demonizing, or making a cheap joke at anyone's expense, and it is this attention to detail that gives this quick-witted study of modern day's generational divide its incisive validity.

Ben Stiller can be very funny playing things broadly (see 2001's "Zoolander" and 2004 "Dodgeball: A True Underdog Story"), but he is even better when a film calls for a more restrained, nuanced performance, as with 2010's "Greenberg" and 2013's "The Secret Life of Walter Mitty." His work here as Josh falls into the latter camp, and his turn stings with humanity even when he is earning a laugh. As Cornelia, who sees herself having increasingly less in common with her friends who have kids and makes a go with Jamie and Darby's younger crowd, Naomi Watts (2014's "St. Vincent") is outstanding. Able to inspire heartbreak and outright guffaws—her forays into hip-hop dancing are sure to go down as some of the funniest moments in film this year—Watts is close to incapable of giving anything other than beautifully complicated performances. In recent years, her gradual reveal of an inspired comedic side has been surprising in the best way possible; she should continue to work in this genre more often. As Jamie and Darby, Adam Driver (2014's "This Is Where I Leave You") and Amanda Seyfried (2014's "A Million Ways to Die in the West") arguably have the more difficult tasks. In lesser hands, their characters could quickly grow unlikable verging on intolerable, but Driver and Seyfried give them the shading necessary to avoid this trap. Jamie and Darby do not always make the best decisions, but they aren't bad people; they simply have a lot of growing to do. Josh and Cornelia do, too, but in different ways.

"While We're Young" provides a fascinating dichotomy of what it is like to be of a certain age, and the unfair expectations that society places on people based on where they think they should be in their lives at any given time. Noah Baumbach's focus on characters and dialogue over plot is necessary, at one with the work of Woody Allen and Nicole Holofcener. When he attempts to add conflict with a key revelation in the third act, the picture temporarily falters with a confrontation between Josh and Jamie that strikes a rare false note. This scene doesn't comfortably fit with all that has come before it, and should have been treated with a more subdued approach. The very ending, too, is questionable; without giving anything away, a major life decision Josh and Cornelia make signals a huge step in their marriage, yet conflicts with the nonconformist message Baumbach has made prior to this about how there is no right or wrong way to pave one's future. "While We're Young" doesn't lead to its conclusion quite as anticipated, but that is also, even in its cursory missteps, part of the appeal. This is a smart, empathetic, sometimes scarily identifiable work from a writer-director who continues to carve a gratifying niche for himself in independent cinema.