Credit: Splash News Online

Though she doesn't turn 18 until Tuesday, Miley Cyrus sure looked like an adult when she was caught in a heavy makeout session at her birthday bash in L.A. Sunday night.

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Clad in a black bra and leather pants, Cyrus leaned up against a banquette as teen star Avan Jogia seductively nuzzled her neck at Trousdale nightclub. See more shots of Miley's birthday bash.

But her steamy smooch with the Victorious actor, 18, wasn't the only thing that had partygoers gawking.

"She was so busy dancing and hanging with her friends," an eyewitness tells, adding that Cyrus and Ke$ha had a dance-off to Pink's "Raise Your Glass" that brought "everyone out on the dance floor."

And don't expect the party to stop anytime soon: Earlier that evening, Cyrus told Access Hollywood that her birthday festivities will continue for at least another week.

"I have a birthday month," she said. "I do the whole month of November. Thanksgiving, cake -- [it's] all about me on Thanksgiving!"