The masses assembled for Billie Eilish’s Glastonbury debut defied categorization. There were teens as well as kids as young as 8 years old. You saw babies strapped into Ergo carriers and grandparents well into their 70s. One thing they did have in common? All looked collectively enraptured by Eilish’s inaugural performance at the massive U.K. festival.

What’s the draw to this prodigious 17-year-old? Eilish’s world is one of wariness and precociousness. She takes on issues of loneliness and adolescent suicide in her songs (“Bury a Friend”), the sort of morose subject matter that attracts a particularly soulful mix of fans — in the case of her Sunday early evening set, a crowd that went bonkers (to filch an English phrase) with rapt enthusiasm. NME described Eilish as a “once-in-a-generation star” following her Glasto performance. We’d simply call that hour of music on the Other Stage life-affirming.

Glastonbury draws some 200,000 attendees each year, and about 40,000 of them were at Eilish’s historic performance on June 30. About that same number were at Miley Cyrus’ show, which was playing concurrently over at the Pyramid Stage. Meaning: Eilish was just as big a draw as Cyrus, who, odd as it might sound given Cyrus is only in her 20s, is already an iconic veteran of the music biz.

Joined throughout her set by her co-writer brother, Finneas O’Connell, on keyboards, Eilish opened with “Bad Guy.” The crowd responded with wanton zeal, breaking into euphoric hysterics, jumping and filming video footage with their iPhones and screeching with sheer joy. Despite technical issues, which Eilish noted resulted in her looking “angry as f–k,” she danced her ass off with wild abandon and engaged the crowd. At one point, she had the entire audience crouch down low and then pop up toward the sky in a sea of orgasmic fervor.

Toward the end of her set she recalled the first time she played London, for a capacity-sized crowd of some 200 people. The highlight of her career, she said. She then looked out at the sea of faces and commented that it was as if “the entire world was staring” at her, and she wanted them to stare at her “straight in the eye.”

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Eilish next asked everyone to put away their phones, because she wanted to be “present,” remarking, “There’s no other time in the history of the world where all of us will have this experience, at this time, at this moment, at this age. We’ll never get this moment back.” Moments later, she launched into her “Ocean Eyes,” a lugubrious yet softer sounding ballad that showcased her impressive vocal range.

“If you absolutely despise yourself, this song is for you,” Eilish followed, introducing “idontwannabeyouanymore.”

It’s this artistic duality of Eilish — fresh-faced California teen; gloomy depressive — that was so mesmerizing to observe, culminating in set closer, and bona-fide pop hit “Bury Your Friend.” Another example of Eilish’s unique duality, it’s a morbid dirge about teenage suicide, but all around, kids as young as 9 and 10 were singing along  gleefully. And you wonder: Do these kids know that Eilish is singing about death? Is this collective expression of impassioned awareness and youthful ruin over the demise of human life the most beautiful thing you’ve ever witnessed or is it deeply disturbing?

With Billie Eilish, it’s a complex and enrapturing combination of both.

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