Madonna -- Ass Cheeks Out ... Squeeze 'Em Easy!

Madonna's ass probably couldn't make it in a Nicki Minaj video these days ... but that's not stopping her from exposing it like it's 1992 again.

The 56-year-old arrived at Staples Center for the Grammys in a sexy matador outfit, and decided to get medieval on photogs -- by hiking up her dress and showing off her aging cheeks.

Later on stage, her butt took on a bunch of half naked masked men with horns.

She keeps getting older, but her ass is ... also getting older. She just hides it well.