Police say the teenager's face was covered when he entered the shop at Lake Munmorah yesterday and threatened a female shop assistant with the weapon.
He allegedly ran away with cash and cigarettes.
A few hours later, the boy's grandparents handed him in at the local police station.
Police allege that the stolen items were later found at the teenager's home.
He has been charged with armed robbery and is expected to face Wyong Children's Court late this month.