Miley Cyrus 21st Birthday -- Strippers, Booze ... and The Lorax

Miley Cyrus turned 21 years old this weekend ... and she celebrated with booze, celebrities, strippers and a guest appearance from a Dr. Seuss character ... and TMZ has the footage.

Miley -- now legal to drink alcohol in the U.S. -- partied at Beacher's Madhouse at the Roosevelt Hotel in L.A. on Sunday night, after the American Music Awards.

We're told Miley rented out the entire theater for the special birthday/AMAs afterparty.

Kelly Osbourne was one of the guests -- and took a pic with Miley with party hats, birthday cupcakes and a bottle of champagne.

Unclear why The Lorax was there ... but he no longer has to speak for the trees that have no tongues ... Miley's got enough tongue for everyone.