Olivia Rodrigo wears a larger-than-life vintage dress to her 21st birthday party

This dress code was legal before it was.

Oliva Rodrigo has always been a fan of vintage clothing, so it’s no surprise that she dug out another sa*sy little dress from the archives to celebrate her 21st birthday earlier this week — and it turns out, her plunging dress is actually older than she is.

“Today is my last day to be able to drink underage (virtually)!!!!” she captioned a set of Instagram photos on Monday.

In the photos, the “Vampire” singer appears wearing a short black dress from Alia’s spring 1991 collection. This simple dress was recently sold – possibly to Rodrigo – for $12,500 on the used site 1st Dibs.

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

Rodrigo’s old dress dropped almost to her navel, with a black leather belt secured below her bust line and another at the waist.

For more Page Six style…

The open sides of the Greek-style dress showed plenty of skin and were held together with small straps.

The “Drivers License” singer wore her brunette hair in a ’90s updo for the party, and switched to a looser style in another photo as she blew out some candles alongside friends like Iris Apatow, Tate McCray, and The Kid LaRoy.

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

Earlier this month, the “Get Him Back” singer rocked another cla*sic look when she wore a sparkling ivory 1995 Versace dress on the red carpet.

Last fall, Rodrigo showed off a cla*sic look when she wore a Betsey Johnson dress covered in floral embroidery to the opening of her “Guts” exhibit — and it turns out the dress was once worn to a concert in Texas in the early 2000s.

Cheers for many more archival moments to come, Olivia.