Crown prosecutor Steve Drake said Melvin Skeete Jr., who is now 18, was sentenced as an adult in Nova Scotia youth court in Sydney.
Drake said Skeete Jr. will be eligible for parole on Dec. 3, 2017, after Judge Anne Derrick credited him with two years served in jail while awaiting trial.
"An adult sentence is a rare thing under the Youth Criminal Justice Act," Drake said in an interview.
"Judge Derrick made a well-reasoned decision on a very complex case."
He said Derrick also lifted the publication ban on the names of the victim and perpetrator in the case.
Skeete Jr. was 16 when he stabbed 17-year-old Brittany Green of Glace Bay 104 times in December 2010. The two had dated for about three months prior to the fatal attack at his grandfather's former home.
"It's not right and to mutilate her body like that," said Theresa Brewster, Green's grandmother, outside court.
"She was such a good person."
Brewster said she supported the judge's decision to remove the publication ban in the case and make public her granddaughter's identity.
"She was like an unknown child," Brewster said.
During his 10-day judge-only trial, Skeete Jr. testified to having no memory of what happened. He said he had spent much of that day popping a variety of prescription pills followed by drinking several shots of alcohol.
He also admitted to a long history of drug and alcohol abuse and rebellious behaviour.
Two psychiatrists who examined Skeete Jr. testified that he suffered from a conduct disorder complicated by substance abuse. However, they disagreed on whether he had the ability to form intent at the time of the attack.
But last August, Derrick convicted Skeete Jr., saying she was satisfied he intended to kill Green notwithstanding "his intoxicated condition."
The defence did not return a message seeking comment.