In the new movie “This is 40,” the writer and director Judd Apatow casts the arrival of life’s four-decade mark as a uniquely brutal crossroads, flagged by sputtering libido, suffocating commitments and curdled dreams.

Judd, buddy, add another eight years, then talk to me. Your body will be even wobblier, your obligations weightier, and time running out more ruthlessly on the gaudiest of your plans. This is 48: in the mail last week, I got a solicitation from AARP. It included a membership card, ready to be activated just as soon as I send in dues, which won’t be anytime soon. And while that premature come-on reflects the group’s relentlessness more than anything else, it’s an accurate reminder that I’m closer to when I’ll quit working than to when I started, my hopes and my hair so fluffy and intact.

That was in my 20s, a period with travails all its own, depicted in another project that Apatow is involved in, as an executive producer. I speak of “Girls,” whose post-college, pre-mortgage heroines flail professionally, fumble romantically and make deeply puzzling wardrobe choices, their outfits emblems of their befuddlement.

The half-hour comedy-drama will begin its second season this coming weekend, and HBO made the first few episodes available to us media types, who have proved that we simply can’t stop gnawing on it. “Girls” is to cultural arbiters what rawhides are to cocker spaniels.

The new episodes immediately reintroduce Lena Dunham’s naked body, which was introduced aplenty in the old episodes. At this fleshy point I could draw it, I could paint it, I could probably reproduce it in clay. Dunham’s character, Hannah, has a new roommate, gay, and a new playmate, Republican. There’s considerable friction, out of bed as well as in.

And there’s a portrait of the period between 20 and 30 as one of peerlessly keen neediness and doubt. You yearn to believe that you’ve figured out the dating game, not yet realizing that it’s eternally unfathomable. You ache for an assurance that you’re pointed in a purposeful direction, but suspect that you’re going nowhere fast. Your desire to project confidence is inversely proportional to your store of it, and you have some really, really bad furniture. I recall, from my mid-20s, a lacquered black table with fake gold accents that cost me next to nothing except, for many years afterward, an undying, unspeakable shame.

We’re a self-absorbed species, and one wrinkle of our self-absorption is our tendency, reflected in our art and entertainment, to believe that there’s no passage of human existence as fraught with perils and as peculiarly significant as the one we just so happen to be going through. Dunham is 26, and “Girls,” which she created, is predicated on the notion that the 20s herald an inimitable sequence of humiliations and unrivaled state of ambivalence. Apatow is 45, and his new movie maintains that to enter your 40s is to encounter an especially messy set of questions about the road taken and the unsmooth pavement ahead. Could any other age compare?

Well, the 30s are no picnic, as we learned in the television drama “thirtysomething,” whose four-season run began in 1987, when its sires, Marshall Herskovitz and Edward Zwick, turned 35. The characters they hatched were roughly their age peers, and turned soul searching into an exercise so vigorous it practically burned calories. Angst overwhelmed them as surely as hormones capsize teenagers.

Speaking of teens, they support whole submarkets of publishing and series of movies dedicated to reassuring them that their pimply predicament is by far the worst: cliques, virginity, trigonometry. But I’m clinging to the conviction that the late 40s are tougher — just try to find a 17-year-old whose left shoulder creaks like mine, and who suddenly has to pitch in thousands toward his apartment building’s new elevators — so that I can congratulate myself for every day I successfully muddle through, every smile I courageously summon.

Then again, this passage isn’t really so insufferable. By 48 you’ve come to know, and quite possibly accept, the well-intentioned wretch that you are, and you most likely have the furniture situation worked out.

The 20s, too, have their perks. You get the freedom of full-fledged adulthood but can make big mistakes without paying huge prices, because there are still so many opportunities ahead for amends.

Dunham isn’t blind to this. In “Girls” she finds the exhilaration amid the mortification. And Apatow’s new movie ultimately understands that being weighed down is just a pessimist’s way of looking at — and talking about — being grounded, which so many of us struggle to achieve. What feel like tethers one day feel like roots the next.

This is 25 and 35 and 40 and, I’ll wager, 50: a matchless kind of awful, a particular stripe of wonderful and just another phase in a struggle that, like our narcissism, is ageless.