Olivia Rodrigo wears a vintage dress older than she is for 21st birthday party

This dress was legal before she was.

Olivia Rodrigo has long been a fan of a vintage ‘fit, so it’s no surprise she dug another sassy mini out of the archives for her 21st birthday celebration earlier this week — and it turns out her plunging dress is actually older than she is.

“today is my last day of being able to underage drink (hypothetically) !!!!” she captioned a carousel of Instagram photos on Monday.

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In the pics, the “Vampire” singer wore a black minidress from Alaïa’s 1991 spring collection, with the barely-there style recently selling — perhaps to Rodrigo — for $12,500 on secondhand site 1st Dibs.

The shopping platform described the dress as “ultra-rare” and “museum quality” and the piece was offered in both black and white at the time of its release.

Rodrigo’s vintage frock dipped down almost to her navel, with a black leather belt cinched under her bustline and another at the waist.

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The open sides of the Grecian-style dress showed off plenty of skin and were held together with tiny straps.

The “Driver’s License” singer wore her brunette hair in a very ’90s updo for the party, switching to a loose style in another pic as she blew out some candles alongside friends like Iris Apatow, Tate McRae and The Kid Laroi.

While her rare vintage dress cost some serious cash, the Grammy winner added a more affordable accessory to her look in the form of a silver Mejuri ring ($68 $41), accenting her features with bold red lipstick.

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Earlier this month, the “Get Him Back” singer wore another vintage look when she wore a sparkling ivory Versace gown from 1995 on the red carpet.

Last fall, Rodrigo showed off a vintage look when she wore a Betsey Johnson slipdress covered with floral embroidery to the opening of her “Guts” gallery — and it turns out the dress was once worn to a Texas prom in the early 2000s.

Cheers to many more archival moments ahead, Olivia.