Madonna Tongues Out with Miley Cyrus -- What, Me Old???

Refusing to age with even a hint of grace, 55-year-old Madonna clung desperately to her youth on stage last night with Miley Cyrus ... like a narcissistic mother trying to be hotter than her daughter.

It's just ... painful.

Madonna and Miley (who's 21 by the way) hit the stage together last night in cowboy get-ups for Miley's "MTV Unplugged" show in Hollywood ... but it felt more like a TLC special.

It's just the latest in Madonna's "I'm still hip" campaign -- in 2012, she named her tour "MDNA" an obvious play on MDMA ... and even once asked a crowd if they'd "seen Molly" ... a sad out-of-the-loop reference to the same drug.

Oh yeah, and a couple weeks ago she referred to her white son as "dis n**ga." Good lord.