A 13-year-old boy has been caught on security video threatening a shop assistant with a knife, at a news agency on the New South Wales Central Coast.

He's been charged with robbery after his grandparents handed him in.

CCTV footage obtained exclusively by 7News shows a young man entered the Lake Munmorah store, his face covered with a red bandanna.

The youth threatened the 20-year-old female shop assistant with what appeared to be a small kitchen knife.

He said "thank you" to the woman, and then allegedly fled with money and cigarettes.

Several hours later, his grandparents took him to Toukley Police Station where he was charged with armed robbery.

FIRST ON 7: A 13-year-old holds up a newsagent with a knife demanding cash and cigarettes.

Officers allege that the stolen property was found inside the teen's home.