Nick Jonas & Olivia Culpo Celebrate His 21st Birthday in Vegas!

Nick Jonas poses with his girlfriend Olivia Culpo as they arrive at his birthday party at XS nightclub on Sunday night (September 15) in Las Vegas.

The youngest Jonas Brother turned 21 years old at midnight! Happy birthday, Nick!

PHOTOS: Check out the latest pics of Nick Jonas

“Gearing up for my 21′st in Vegas T minus 2 hours and 40 mins till party time,” he tweeted.

Nick was joined by his brothers Joe and Kevin, and their significant others Blanda Eggenschwiler and Danielle Jonas.

20+ pics inside of Nick Jonas and Olivia Culpo in Vegas…