When Mattisse Jackson's mother wouldn't let him and a friend smoke cannabis in the house, he threatened to stab her and smashed her belongings.
The victim was at home with Jackson on May 15 at the Castlecliff address when an argument began over the smoking issue. Jackson began yelling and swearing at his mother, the Whanganui District Court heard on Tuesday.
"I'm going to put some knives into you," he told her.
The victim heard "smashing sounds" as she left the address, police prosecutor Sergeant Stephen Butler said.
She returned to find the modem, vacuum cleaner and computer broken.
Jackson told police "she makes fun of me, I've had it all my life".
He pleaded guilty to wilful damage and behaving threateningly.
Judge Dugald Matheson remanded him to June 30 for sentencing. The matter has been referred to Restorative Justice.