The Jonas Brothers are all grown up.

The youngest member of the boy group celebrated his 21st birthday in Las Vegas over the weekend -- where Nick Jonas shared some serious PDA with his gorgeous girlfriend, Miss Universe Olivia Culpo.

The festivities kicked off Saturday night at Andrea's at the Wynn Las Vegas -- where the couple was joined by brothers Joe and Kevin and their +1s, Blanda Eggenschwiler and Danielle Jonas, respectively. 

The celebrations continued on Sunday night, where Nick had his first legal drink (a glass of champagne) at Botero steakhouse. 

The trio and a group of 20 friends then hit up XS nightclub, where they had a booth next to DJ Avicii.

But the most important question: Who gave Nick the best gift?

Olivia reportedly got her man a necklace for his special day, while brother Joe gave him a pair of Frank Sinatra's tap shoes! How's that for unique?

Check out more photos from the weekend-long festivities above. Happy Birthday, Nick!