KYLIE JENNER MY INSANE 21ST BIRTHDAY!!! The Gang's All Here

Kylie Jenner can legally drink -- and she's a billionaire -- which are 2 good reasons why there were 2 insane parties Thursday night that not only brought out the Kardashian brood in full force, but reunited Scott with Kourtney and Caitlyn with Kris ... at least for one night.

You gotta see the video. Hoards of photogs pounced on Craig's in WeHo Thursday night, as Kris, Kim, Kanye, Kendall, Kourtney, Scott and Khloe made their way in.

Caitlyn, who has been estranged from most of her former fam, made a rare appearance ... partly because she still has a relationship with Kylie.

It was more than a family birthday celebration ... Dave Chappelle, Chandler Parsons, Kevin Durant, Paloma Ford, Meek Mill, Chris Brown and French Montana were on hand for the bash. Bella Hadid and The Weeknd were also there together.

It didn't end at Craig's. The party moved to Delilah, where everyone cut loose. Chappelle grabbed a mic and sang, "Killing Me Softly."

BTW ... Tristan Thompson was nowhere in sight. He was in Toronto at an epilepsy fundraiser to support research.

tmz