Vyleen White was murdered by the boy, then aged 16, in a car park at Redbank Plains shopping centre, west of Brisbane in February 2024.
The 70-year-old's death was a catalyst for the Queensland Liberal National government's controversial "adult crime, adult time" laws, ensuring juveniles face at least 20 years in custody for murder.
However they are not retroactive, meaning the teen was on Thursday sentenced under Queensland's previous laws after earlier pleading guilty to murder.
Chief Justice Helen Bowskill described the boy's actions as "cowardly and callous" when she handed down her sentence.
The teen - now 17 and who can't be legally identified - not only took one life but destroyed many others when he murdered Ms White while she was holding up her hands and backing away, she said.
A little girl witnessed the "senseless" attack.
"(Ms White) was a person who mattered greatly to her family and her community," Justice Bowskill said.
"All lives matter. They are precious. They are not to be wasted by selfish and senseless acts of violence."
The teen quickly approached Ms White before he demanded her car keys then inflicted a 17cm-deep knife wound that was fatal.
The teen stole Ms White's 2009 Hyundai Getz hatchback and drove it to a nearby residential area where he showed it off to other teens.
"Taking Mrs White's life has caused immense, indeed catastrophic pain and suffering to her family," Justice Bowskill said.
The little girl who witnessed the stabbing was very brave in seeking help from bystanders in the horrendous circumstances of the murder, she said.
Ms White's husband Victor and his three daughters wept as they watched the sentencing via a video link to a nearby courtroom.
The grandmother had been married for 50 years and was a companion to her husband, helping him with his worsening eyesight, Justice Bowskill said.
But the teen was remorseful and had spared Ms White's family the trauma of going through a trial by pleading guilty, she said.
Crown prosecutor Chris Cook pushed for a sentence of 17 to 18 years, saying the teen's offending was significantly aggravated by him covering his face with a mask and the murder was witnessed by a young girl.
The teen was also on bail for armed robberies, including an incident where a man was injured with a knife.
The teen's barrister Matthew Hynes had pushed for the killer to be released before he turned 25 because the murder was not premeditated.
The teen also pleaded guilty to car theft and three counts of stealing.
He stared straight ahead and did not visibly react as his 16-year sentence was handed down.
The teen will be released from detention after serving 60 per cent of the 16 years.
He has already served one year and nine months on remand, leaving him likely to be released in late 2033.
