After her powerful performance at the American Music Awards, Miley Cyrus kept the party going at Trousdale to celebrate her 18th birthday. The official day isn't until tomorrow, but that didn't keep her family and friends from coming out to party with Ms. Miley!

Aside from Ke$ha, Kelly Osbourne, mom, Tish and look-a-like sister Brandy, Miley's LOL co-star Demi Moore even popped in for a bit.

Sources from inside the club tell us that Miley, dressed in leather bikini top, was getting "hot and heavy" with Nickelodeon star, Avan Jogia ... who she was seen with last month.

Watch out, world ... as of midnight, the former Disney star is an official adult.