Miley Cyrus' Birthday Neck Suck

Wearing a leather bikini top, underage pop star Miley Cyrus proved she can't be tamed ... by getting kissed on by some guy at her post-AMAs/pre-18th birthday party at Trousdale at around 2AM this morning.
The mystery dude sprawled on top of Miley may be her rumored new boyfriend, Nickelodeon star Avan Jogia ... but it's pretty hard to tell for sure.

Miley officially turns 18 on Tuesday.