Less a story about growing up and more one about four friends’ inability to do so, “The Last Kiss” is uninspired Pablum. Approaching their 30th birthdays, four longtime buddies are both woefully adolescent and prematurely middle-aged — perhaps the great paradox for Generation X-ers with a Y chromosome.

While older movie critics seem to find a 20-something “in crisis” precocious, they fail to understand how legitimately and existentially angsty being a young adult in the 21st century can be. We have abbreviated childhood and drawn out adolescence so much that the onset of adulthood is confused and nebulous at best. But there is a difference between having an arguably premature midlife crisis and refusing to grow up at all, and the unsympathetic characters in “Last Kiss” fall into the latter category.

Nearing the start of his fourth decade, Michael (Zach Braff) has a pregnant girlfriend, Jenna (Jacinda Barrett), whom he contemplates marrying. But despite the lovely, blossoming Jenna, his job at an architectural firm and the close friends he’s had since childhood, Michael is terrified by the predictable trajectory of his life and “falters,” as he so euphemistically calls it, into the arms of 20-year-old flautist Kim (Rachel Bilson of “The OC” fame), whom he meets (where else?) at a wedding.

Although Braff’s wide eyes and weak chin are intended to garner sympathy, and his character’s feigned hesitation to Kim’s overly aggressive advances is supposed to suggest guilt, Michael is pathetic and annoying in his aversion to responsibility.

Meanwhile, Michael’s equally troubled (and immature) friends face romantic problems of their own. Michael watches as the marriage of his closest friend and coworker Chris (Casey Affleck) collapses under the strain of parenthood. Izzy (Michael Weston) remains fixated on the long-term girlfriend who dumped him, and decides a South America adventure a la Che Guevara is the answer (although Izzy’s will be the RV Diaries). And the Casanova of the bunch, Kenny (Eric Christian Olsen), bolts as soon as his casual but regular sexual partner expresses interest in anything more than carnality.

In all fairness, though, the boys of “Last Kiss” are not the only characters who are difficult to identify with. Kim is little more than a caricature, and one that was poorly sketched at that: She is obnoxiously perky, sexually manipulative and desperate. Jenna, while more mature than her wayward boyfriend and at times almost likable, is reductive and unabashedly a daddy’s girl. Her mother, Anna (the shamefully underused Blythe Danner) is the most complex of the female characters, having long suffered under the cold sarcasm of her pychiatrist husband (Tom Wilkinson), but even she tends toward the stereotype of the emotionally volatile, melodramatic wife.

All of these subplots are inexpertly arranged as foils to the Michael-Jenna saga, lending disjointedness to an already lackluster story.

Expected by many to be a further exploration of the themes introduced in Braff’s “Garden State,” “The Last Kiss” is instead a rumination on the fallacy of the romantic ideal and a portrait of arrested development. Hollywood has tried to broach such subjects before (recently in “The Break Up” and “Failure to Launch”) but has been, and remains, unsuccessful at offering any valuable insight. And while director Tony Goldwyn and writer Paul Haggis are to be applauded for at least attempting honesty, it is difficult to reconcile the bromidic dialogue of “Last Kiss” with the knowledge that Haggis also wrote the screenplays for two best picture Oscar-winning films (“Crash” and “Million Dollar Baby”).

One could at least hope that the “Last Kiss” soundtrack, compiled by Zach Braff, would be as good as its “Garden State” counterpart. And although “Last Kiss” is equally rife with indie ballads by the likes of Coldplay, Snow Patrol and Imogen Heap that lament infidelity and love lost, a few good songs do not make up for the triple threat of subpar writing, acting and directing.