A vicious Brooklyn teen threatened his 93-year-old grandmother with a knife and then beat her with his fists, cops said yesterday.

Micah Miller, 19, attacked the elderly woman inside an Ocean Avenue apartment in Flatbush Wednesday and was in court last night awaiting arraignment.

“He threatened her with a large knife and began punching her about the face and body,” a police source said.

The injured granny was taken to Kings County Hospital in critical condition with a collapsed lung and broken ribs, police sources said.

Miller was set to be arraigned on charges that included felony assault, menacing, and criminal possession of a weapon.