TORONTO -- A man convicted of murdering four family members was only 17 years old at the time of their deaths -- not 18 and a half -- the man's lawyer argued Thursday as he asked Ontario's highest court to grant his client a new trial.

Hamed Shafia's defence lawyer asked the Ontario's Court of Appeal to admit fresh evidence which he said proves his client was a youth when his three sisters and another family member were found dead in a car at the bottom of a canal.

"We have evidence that is reasonably capable of belief," Scott Hutchinson told a panel of three judges. "In my submission you must give effect to the fresh evidence, set aside the conviction and order a new trial."

Shafia and his parents were convicted in January 2012 of four counts of first-degree murder in the deaths of his sisters Zainab, 19, Sahar, 17, and Geeti, 13, and his father's first wife in a polygamous marriage, 52-year-old Rona Amir Mohammad.

The Shafia family was originally from Afghanistan but fled at the outbreak of war in the country and eventually immigrated to Canada. Their case made international headlines and triggered a criticism about the so-called "honour killings."

The victims' bodies were found on June 30, 2009, in a car at the bottom of the Rideau Canal in Kingston, Ont.

The Crown at the trial asserted the murders were committed after the girls "shamed" the family by dating and acting out, and Amir Mohammad was simply disposed of.

The trial judge described the killings as being motivated by the Shafias' "twisted concept of honour."

Hutchinson said that at the time of the trial, Hamed Shafia didn't know that he could be a year younger than he thought he was and neither did his father or his trial counsel.

In explaining the issue, Hutchinson urged the court to be mindful of a "casualness" associated with birth dates in Afghan and Middle Eastern communities.

The issue with Shafia's age only came to light after he and his parents had been sentenced to life in prison, Hutchinson said.

It happened when his father wanted to transfer property in Afghanistan -- where Shafia was born -- and asked someone in the country to prepare necessary paperwork, court heard.

That person discovered Shafia's original Afghan identity document -- known as a "tazkira" -- which recorded his birth date as Dec. 31, 1991 -- making him a year younger than initially thought, Hutchinson told the court.

To resolve the discrepancy, a certificate of live birth was obtained from an Afghan ministry, which turned out to have the same date, and the Afghan government also issued a document confirming the tazkira, Hutchinson said.

The three documents together are key evidence, Hutchinson said.

"The real issue for the court is whether or not those documents, collectively, are reasonably capable of belief," he said, noting that the Afghan embassy in Ottawa examined the documents and had no concerns about their authenticity.

Hutchinson admitted that Shafia had other identity documents, including ones issued in Canada, that carried a different birth date, but said the newly discovered Afghan documents nonetheless raised significant questions.

"There would be a temptation for some to think of this as a technicality," he said. "Age in so far as it relates to young people is not a technicality, it is a principle of fundamental justice....We do not treat young people the same way we treat adults."

An adult convicted of first-degree murder faces life without parole for 25 years, while a young offender, when sentenced as an adult, faces a maximum of life without parole for 10 years.

If a new trial for Shafia before a youth court could not be obtained, Hutchinson asked the panel of judges to order that Shafia be granted a new sentencing hearing under the YCJA, or order that he should have his sentence reduced.

The arguments made Hutchinson added a new twist to a joint appeal being made by Hamed Shafia and his parents that contests their convictions.