The Shannon Twins -- Karissa and Kristina -- turn 21 today and the Playboy Playmates got the party started just after midnight Friday, RadarOnline.com can report.
Champagne, vodka and beer all flowed non-stop as The Girls Next Door stars started the fun at NYC's Via dei Mille and then moved on to the Greenhouse nightclub where they partied into the early morning hours.
Not present were the girls' former good friends Heidi Montag and Spencer Pratt.
Karissa told RadarOnline.com this summer that she'd made a "sex tape" with Heidi Montag that Spencer had stolen and was trying to sell.
Pratt never produced the tape -- after flapping about it for weeks -- and he and Heidi have since called off their sham divorce proceedings.