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For the second time this year, writer-director Noah Baumbach finds great humor, even farce, in a close look at generational divides.

This spring, his movie “While We’re Young” lovingly skewered a 40-something couple caught up in a kind of love/hate relationship with a 20-something couple. And now, in his new movie, “Mistress America,” Baumbach pairs a free-spirited 30-year-old woman with her future 18-year-old step-sister whose life has yet to begin. What starts out as hero-worship on the part of the college student gradually turns into an insightful character study about a woman who does everything and nothing.

Actress Lola Kirke plays Tracy, a homesick freshman at Barnard College in New York City. At her mother’s encouragement, she reluctantly seeks out her soon-to-be step-sister who also lives in the city, the 30-year-old Brooke, played by Greta Gerwig (Tracy’s mom is marrying Brooke’s dad).

Tracy is, at first, blown away by Brooke’s whirlwind life. She’s a freelance interior designer, a spin instructor, a singer in a band, and a budding restaurateur. She’s an independent, self-made woman, fending for herself in the big city.

“Being around her was like being in New York City,” the trailer notes.

Brooke seems to know everybody and loves showering her new sister with words of wisdom and advice.

“There’s no cheating when you’re 18,” she says. “You should be touching each other all the time.”

Tracy is convinced that Brooke is leading exactly the kind of life any young woman might hope for. But she also sees some of the cracks in her glittering facade. Tracy decides to write a short story about Brooke, a short story she somewhat pretentiously calls “Mistress America.”

Her story, of course, plays a part in the film’s farcical climax that includes not only Tracy, Tracy’s love interest and Tracy’s love interest’s girlfriend, but also Brooke’s rich ex-boyfriend, Brooke’s one-time best friend and now hated rival who married that ex-boyfriend, and a couple of stolen cats. The only thing that’s missing from this screwball comedy is a few slamming doors.

But despite the laughs, what makes this movie stand out is its razor-sharp portrayal of a very specific type of person, a recognizable type that we don’t very often see in movies. Brooke is a natural born charmer who’s been on the cusp of adulthood for so long she can’t figure out how to move beyond that cusp. As displayed in a conversation with a male friend.

“You’re funny, because you don’t know you’re funny,” he says.

“I know I’m funny,” Brooke responds. “There’s nothing I don’t know about myself, that’s why I can’t do therapy.”

Therapy might do Brooke a lot of good, but when self-confidence and self-delusion are so closely entwined, it’s tough to separate them. Fortunately for us, her struggles are endearing and yes, funny &#8212 even if she knows it.