Judge Harvey said they made off with $269 in cash.
Steen and his accomplice committed the most serious offence in a Te Kopuru house on June 12, the judge said.
Armed with a hammer, the pair broke into the house and approached a 67-year-old woman as she lay on a couch.
The woman was dragged through a hall, assaulted, and as she got up was again attacked, before Steen tied her legs and gagged her with a tea-towel.
"You struck her purely on a gratuitous basis," Judge Harvey said.
"She wasn't struggling, she wasn't stopping you and when she got up, you hit her again."
When arrested, Steen said he committed the offences because he was bored.
Crown prosecutor Davis Stevens said Steen's behaviour was surprising, given that he had no previous convictions. Mr Stevens said violence was inherent in all the charges and that both victims were vulnerable.
Steen's lawyer, Julie Young, said he lacked skills to deal with difficulties in life.
His accomplice is being dealt with by the Youth Court.