Joshua Harmon is the master of the rant. He earned that accolade with his last play, “Admissions,” in which a white prep-school student delivers a lengthy screed on the injustice of his being rejected by Yale after his less worthy biracial friend has been accepted at the Ivy League school.

Harmon is a master of the rant for two reasons: The student’s very length speech in “Admissions” is alternately funny, absurd, logical, glib and intensely heart-felt. The other reason is that Harmon repeats that success not once but several times in his new play, “Skintight,” which opened Thursday at Roundabout’s Off Broadway Laura Pels Theatre.

The unfairness he documents this time has nothing to do with race and everything to do with age. Young people are attractive (well, some of them are) and old people are not. As described by two of Harmon’s characters, even the middle-aged have lost their sex appeal and grow hair in weird places.

Also Read: 'Othello' Theater Review: Corey Stoll Makes Iago a Mesmerizing Villain

Since Jodi (Idina Menzel) is a fortysomething woman, she hasn’t yet sprouted hair on her knuckles or in her ears. But her fiftysomething ex-husband has, and so it’s humiliating that she’s the one being dumped for a younger woman, a 24-year-old spinning instructor with breast implants.

We learn all about this split and her ex’s encroaching second wedding in the opening scene of “Skintight.” Jodi just flew across the country, she’s tired and cranky and she made the trip more to escape her ex in Los Angeles than to see her very successful fashion-designer father, Elliot (Jack Wetherall), who would rather not have his daughter there to help celebrate his 70th birthday.

The most tantalizing aspect of Harmon’s new play — besides the fact that it’s very funny and poignant and well-crafted — is that the playwright explores the unfairness of Jodi’s predicament not by dramatizing her failed heterosexual relationship; instead, he lets us mull over the supposedly successful same-sex affair that Elliot is having with a 20-year-old man named Trey (Will Brittain), who is very buff and in one scene walks around in a jock strap — to the dismay of Jodi and the rapt attention of her own out 20-year-old son, Benjamin (the wonderfully nebbish Eli Gelb).

Also Read: 'Sugar in Our Wounds' Theater Review: The Earth Grandmother Steals the Show

And there’s something else that Harmon stresses without ever having any of his characters talk, discuss or rant about it: Ageism is very much a guy thing, particularly an affluent guy thing, whether the perpetrators are gay or straight. Women are on the receiving end — and sometimes men are too: just ask Elliot’s ex-lover turned manservant, Jeff (Stephen Carrasco), who doesn’t get many lines but is expert at impersonating a slow-walking phantom to Wetherall’s vampire of a man.

There’s a scene at night where Elliot, standing on the upstairs railing of Lauren Helpern’s well-appointed and extremely sterile living room set, calls to his “partner” downstairs in order to get him back into bed and, more important, prevent a midnight tryst in front of the flickering TV set between the two 20-year-olds. Elliot’s pointed request is as creepy as it is smart. Pat Collins’ lighting sets the tone perfectly.

The beauty of Daniel Aukin’s direction is that he keeps us guessing about each and every character’s real motives until the end, and even after the curtain has dropped.

Also Read: 'Pass Over' Theater Review: When the White Man Is an Alien From Another Planet

“Skintight” might be the first great post-gay gay play. Most of the characters are homosexual, and not just incidentally so. But here’s the “post” part: Not one of them sees himself as a victim, unless by victim you mean that they’re trapped by a toxic brew of hormones and testosterone that force them to value beauty over substance, lust over love.

Jodi definitely claims victim status, but she is far from guilt-free, at least not as portrayed by Menzel. It’s weird to call hers a breakthrough performance when this actress already has a Tony Award to her credit, but the nuanced comic timing she brings to her distressed character is masterful. Menzel also softens Jodi’s victimhood by making her pampered to the extreme. Just check out her total lack of respect for Trey, who gives twinks a good name — at least as portrayed by the talented Brittain.

The word “pampered” is mentioned only once in “Admissions,” yet it’s that other sin at the heart of Harmon’s plays. His characters don’t have truly pressing problems. They’re not hungry, homeless or about to be deported. What they do have is the luxury of thinking that tomorrow’s Botox treatment will bring them happiness.