Detective Nicole Monroe said the woman, 66, was discovered unconscious in bed, The Baltimore Sun reported. She was reported to be in critical condition.
Prosecutors have not said whether they would seek an adult trial for the boy.
He allegedly told police he was getting ready to go to school and became angry at his grandmother. He said he hit her several times with the hammer.
Handler slams Piers Morgan: 'You're a terrible interviewer'
McPhee, Cokas 'working on their marriage' after affair