Set aside for a moment the macro-issue sociopolitical echoes that could maybe, if negotiated brilliantly, make a 2024 production of this Vietnam-era protest musical seem fresh as a flower in a reservist’s rifle barrel: I turned up for the opening of Signature Theatre’s staging just off a conversation with a teenage nephew whose hoop dreams seem to be fading and whose relationship just evaporated, which may be why the essential sweetness and fragility of “Hair’s” struggling individual adolescents is what registered most with me this time around.

So thoroughly of its time are the show’s once-radical specifics — it’s all in on free love, psychedelic drugs, and men figuring out that OMG, they can like each other — that it really has no business working anymore. Yet in an America whose kids seem uncertain about their place and almost frantically eager to find better ways of being in the world, Matthew Gardiner’s exuberant and openhearted production sure does — and it bit me right in the soft spot where I usually store up my bile.

You know the big songs — “Aquarius,” “Let the Sunshine In,” “Good Morning Starshine” — and the Signature Theatre cast, backed by Angie Benson’s bangin’ eight-piece band, sells them with all the heart and the lung power you’d expect in a spring-tentpole production from an American musical-theater powerhouse. The physical staging more than measures up, too, with costumes by Kathleen Geldard so apt they might be made entirely of hemp and patchouli, plus a jam-packed set (by Paige Hathaway) that compresses the proceedings toward the audience. Activated by tightly integrated video and lighting schema (via Patrick W. Lord and Jason Lyons), the frame can make the evening feel like a shared hallucination unfolding inside one of Peter Max’s lunchbox radios.

I always forget, though, that over two acts that could easily be a single, tighter one, the “Hair” writing team (Galt MacDermot, Gerome Ragni and James Rado) packs in a host of disposable pastiche numbers that don’t earn their time onstage. (Looking at you, sneering satire of flag-waving yokels, and maybe even you, uninspired riff on federal agencies with imposing initials.) That these hang like solo clothespins upon the barest line of plot — will the sensitive but hardly iconoclastic Claude join the other men of his “tribe” in burning his draft card and stiff-arming the status quo? — doesn’t help speed the show along.

So even in a first-class commercial revival like the 2007 Diane Paulus staging that started in Central Park and later played the Kennedy Center on its way around the nation, “Hair” can feel like an exercise in diminishing returns. The sonic explosion of “Aquarius” opens the proceedings with a sense of searchingly honest wonder, and the downright hymn-like ecstatics of “Let the Sunshine In” send the audience out feeling like it’s been taken properly to church, dubious as “Hair’s” counterculture warriors might feel about the simile. In between, though, those lesser tunes come and go without much impact, and the ‘60s-flavored shots they take at the mid-century monoculture don’t feel as tart as they once must have.

The gems among the gravel are numbers that sketch piquant character moments in a show that doesn’t offer many: “Where Do I Go,” for the uncertain Claude; the exquisite “Frank Mills,” in which a young woman rhapsodizes a connection that’s probably been missed but might still be just possible; and “I Got Life,” a wan body-parts listicle on the page but an irresistible carnal bop in the hands of Signature’s sexy, sweaty gang of theater kids.

Who are, by the way, sensational — bendy, bouncy, bountiful in every shape and shade imaginable, in sync and on point and eager to connect as they move through Ashleigh King’s vivacious choreography, bright-visaged and sly-eyed and just joyously alive. In the hands and on the bodies of an ensemble this tightly drilled and audaciously confident, even a messy old museum piece like “Hair” can get up and move.

Hair, through July 7 at Signature Theatre in Arlington, Va. About 2 hours 30 minutes including intermission. sigtheatre.org.