Open this photo in gallery:

Canada’s top court in two related decisions on Friday provided added detail on how the laws regarding sentencing youth should be interpreted by lower courts.Amber Bracken/Reuters

The Supreme Court of Canada has made it more difficult for judges to sentence youth offenders convicted of serious crimes, such as first-degree murder, as adults for their actions.

The top court in two related decisions on Friday provided added detail on how the law should be interpreted by the lower courts.

In Canada, few young people are convicted of such serious crimes, and thereafter sentenced as adults, but the decisions are still likely to represent important precedents in how lower courts decide these cases in future.

The Youth Criminal Justice Act revolves around the principle of a young person’s presumed “diminished moral blameworthiness” – that people younger than 18 may not make decisions like an adult and in general should not be held to the same standard in criminal proceedings.

For a young person to be sentenced as an adult, the legislation indicates that Crown prosecutors must rebut the presumption of diminished blame, as well as proving a youth sentence is insufficient to hold the person accountable for their crime.

In the primary ruling on Friday written by Justice Nicholas Kasirer, in a 7-2 majority, the top court said the Crown must prove beyond a reasonable doubt that a young person does not merit the presumption of diminished blame for their crime on account of their youth.

The top court also said a serious crime, such as first-degree murder, shouldn’t be a main factor to lead judges to impose adult sentences.

Justice Kasirer cited specific considerations to weigh about a young person’s relative maturity such as “impulsiveness or bravado that indicates immaturity, and whether planning reveals a level of sophistication and foresight that aligns with adult‑level reasoning.”

Nader Hasan, a lawyer at Stockwoods in Toronto and defence counsel on the main case decided Friday, said the Supreme Court has restored the basic presumption that youth offenders should be sentenced as youths, with the focus on rehabilitation.

“Children are children even when they have done something terrible,” he said.

Mr. Hasan was counsel on the case known as I.M. It served as the lead decision issued by the Supreme Court on Friday, one of two cases heard together last October.

The I.M. ruling landed at an epic length of about 40,000 words, both the majority decision and the dissent, together nearing the size of a typical book.

In I.M., Justice Kasirer and the majority provide their broader legal thinking, including the standard of beyond a reasonable doubt on whether to push a youth sentence to the adult level. The legislation doesn’t include such wording.

Instead, the law says a youth justice court can impose an adult sentence if it is “satisfied” that the Crown has rebutted diminished blame, alongside the second factor about a sentencing being long enough to hold the person accountable.

Using the new framework, the Supreme Court overturned I.M.’s adult sentence and imposed the maximum youth sentence. I.M. was 17 when he and co-assailants beat and stabbed a victim who died from his wounds.

The difference in punishment is significant: I.M.’s adult sentence was life in prison with no chance of parole for 10 years; his youth sentence levied by the Supreme Court, the maximum, is 10 years, with six years in custody and four years of conditional supervision.

In a second case, known as S.B., the Supreme Court applied its legal framework set out in I.M. and upheld S.B.’s adult sentence for first-degree murder of life in prison with no chance of parole for 10 years.

Lawyers for S.B., despite their client’s loss, said the decisions are a demarcation in the sentencing of young people for serious crimes.

Laura Remigio and Kristen Dulysh of Derstine Bariteau in Toronto said that in recent years Crown prosecutors often sought an adult sentence if a young person had been convicted of first-degree murder in youth court.

The new higher standard that youth court judges have to consider could have a “massive impact on the application of this test in future cases,” said Ms. Dulysh.

Annamaria Enenajor, a Toronto lawyer at Ruby Shiller Enenajor who represented an intervener in the cases, the Queen’s Prison Law Clinic, said the latest thinking from the Supreme Court is consistent with its established views on youth criminal justice.

In particular, a 2008 ruling known as D.B., written by then-justice Rosalie Abella, focused on the principle of diminished moral blameworthiness because of a young person’s age and factors such as reduced capacity for moral judgment.

Ms. Enenajor said Friday’s decisions highlight a philosophy behind youth justice in Canada that “believes strongly in second chances” – punishing young people for their crimes but not as severely as adults.

“It’s really about the offender and their maturity and capacity for independent judgment,” said Ms. Enenajor.

In the dissent of the 7-2 I.M. ruling, co-written by justices Suzanne Côté and Malcolm Rowe, the focus was on the wording of the legislation, specifically that a youth court must be “satisfied” with the Crown’s rebuttal of the presumption of diminished blame.

The two dissenting justices disagreed with their colleagues’ higher standard of beyond a reasonable doubt and pointed to Parliament’s legislative history that considered the inclusion of such wording in the law but ultimately left it out.