If it's a youth sentence, which the vast majority are, that's a sentence of 10 years at age 15. It's already clear in the YCJA that for a youth sentence, they can only serve it in a youth custody facility. The rule is generally that the young person would stay there until the age of 20.
However, at the age of 18, there can be an application to a court to ask whether that particular young person should be moved up into adult custody. The judge will look at all the circumstances and will hear from the family, the youth, the prosecution, and the corrections people. The judge will also consider what is in the best interests of the young person, but there is the possibility that the young person could be moved up at age 18.
If they're serving a sentence and still in custody at the age of 20, they can then be moved up without going to a court, but they do not have to be moved up. There is still discretion for the province to keep that young person in the youth facility. For instance, if only a few months or a short portion of the period of the sentence is left, they wouldn't move the young person up to adult custody.