The Youth Criminal Justice Act does not permit the joint trial of one individual on both adult and young offender indictments.
The Youth Criminal Justice Act (YCJA) is the law that governs Canada’s youth justice system. It applies to youth who are at least 12 but under 18 years old, who are alleged to have committed criminal offences
In R. v. S.J.L., 2009 SCC 14,  1 S.C.R. 426, the Supreme Court of Canada held that the YCJA does not permit the joint trial of a young person and an adult. The principle upon which S.J.L. rests precludes the joint trial of a young offender indictment and an adult indictment involving the same accused: R. v. P.M.C, 2016 ONCA 829.
Writing for the majority in S.J.L., Deschamps J. held, at para. 72, that “it can be stated definitively that ‘[t]he youth justice system is separate from the adult system, with separate courts, judges and rules’ (L. Tustin and R. E. Lutes, A Guide to the Youth Criminal Justice Act (2005), at p. 29).” She added, at para. 74, that “Parliament intended to establish a youth criminal justice system that is hermetic, and completely separate from the system for adults, and thus to make it impossible to hold joint trials of adults and young persons.” The rationale for this separation was explained, at para. 75:
“[T]he effect of the objectives of the [YCJA] is that the judge is asked to favour rehabilitation, reintegration and the principle of a fair and proportionate accountability that is consistent with the young person’s reduced level of maturity. As for the adult criminal justice system, it places greater emphasis on punishment. There is no doubt that how the judge conducts the trial will reflect these different objectives. It would be much more difficult to maintain an approach favourable to a young person if he or she were being tried together with an adult, and the presumption of diminished moral blameworthiness to which the young person is entitled could be undermined as a result.”
While those statements were made in the context of a case involving the joint trial of distinct adult offenders and young offenders, they apply with equal force to a joint trial of one individual on adult and young offender indictments. Indeed, it would be even more difficult for a trial judge to maintain the favourable approach and presumption of diminished moral blameworthiness towards an accused as mandated by the YCJA if the accused were being simultaneously tried for adult offences. Even where the accused is an adult when the young offender charges are tried, he/she is entitled to be tried and judged on the legal standard that applied to him/her at the time he was alleged to have committed these offences.
S.J.L. establishes that the maintenance of the YCJA standards requires separate and discrete consideration, hermetically isolated from the legal standards that apply to adults.