He was charged as a juvenile, so his punishment could entail up to 18 months in a juvenile facility. A judge is due to decide Dec. 6.
The boys are charged with second degree assault.
They're accused of dropping a shopping cart off a fourth floor mall walkway in East Harlem.
The cart crashed on the head of real estate broker Marion Hedges who is still in the hospital in serious condition.
The second 12-year-old boy pleaded not guilty.