Good 4 her! Olivia Rodrigo turns MSG into a giant slumber party in sold-out headlining debut

They sure do mature fast in pop.

When Olivia Rodrigo took the piano to sing her smash debut single “Drivers License” on Friday night — during the first of four sold-out shows at Madison Square Garden on her “Guts” World Tour — it was almost as if she had outgrown the song.

Just three years ago — when the breakup ballad floored it to No. 1 in 2021, putting the pop ingenue on the fast track to three Grammys — Rodrigo had yet to graduate from Disney’s “High School Musical: The Musical: The Series.”

So singing about the rite of passage of getting your driver’s license made her feel just like the 17-year-old that she was when she wrote, recorded and released her first of three chart-toppers.

Flash forward to April 2024, and Rodrigo has made Disney a distant memory, leaving “High School Musical” in 2022, and graduated from playing Radio City Music Hall on 2022’s “Sour” Tour to making her headlining debut at the big daddy of New York arenas.

And while she wasn’t even old enough to vote when she dropped “Drivers License,” she’s now 21, coming of drinking age on Feb. 20.

“I had my first sip of alcohol in my life on my 21st birthday,” said Rodrigo, while making a winking reference to her grandmother being in the audience.

Before launching into the “Guts” closer “Teenage Dream,” she continued, “I’m 21 now and just so happy. I feel like I know myself so well, and I think if [I could give] any advice to my 18-year-old self, I would tell her that growing up is f—king awesome.”

Certainly, the vibe on Friday night was totally awesome, as hordes of screaming and loudly singing girls — many in mini-skirts and Dr. Martens boots a la Rodrigo; some accompanied by their parents  — turned the Garden into one giant slumber party.

After she came rocking out in a sparkly silver two-piece to the “Guts” guitar jams “Bad Idea Right?” and “Ballad of a Homeschooled Girl,” Rodrigo quickly veered into “Vampire,” the piano ballad that became her most recent No. 1 single last July.

That she would do one of her biggest hits so early in her show — and a slow song, no less — was a sign of how confident Rodrigo was in her lesser known material.

With only two studio albums — 2021’s “Sour” and 2023’s “Guts” — she performed for an 1 hour and 45 minutes without it feeling like there was too much filler, save for one slowish patch mid-show.

No doubt, this is one designed for the diehards — but clearly there are plenty of those. (And, hey, at least the casual adult fan didn’t have to worry about a line at the bar.)

With her voice and energy never wavering, Rodrigo — bringing to mind a mash-up of Billie Eilish and Avril Lavigne, with some of the ’90s alt-rock attitude of opening act the Breeders — was alternately vulnerable (“Logical” and “Enough for You,” riding a crescent moon across the arena) and powerful (“Brutal” and “Obsessed”).

And there was even a relatable fangirl moment with surprise guest Noah Kahan on “Stick Season.”

After ending her set with “All-American Bitch,” Rodrigo — who will return to MSG on Saturday, Monday and Tuesday — kept her punk-pop pout firmly in place for the raucous encore of “Good 4 U” and “Get Him Back!”

Two years after Rodrigo performed “Deja Vu” and “Uptown Girl” with Billy Joel at his house, the Garden was all hers.