That is not entirely how it works. I can explain it for you. When a young person commits an offence, sometimes they are arrested some time after the offence, sometimes immediately after the offence. Crown counsel can ask that adult sentencing apply. The proceedings may last some time and that is what sometimes means that the judge makes their decision after the young person reaches the age of 18 years. That is why it is very rare for a person under the age of 18 years to be sentenced as an adult, in Quebec in any event. I can't speak for the rest of Canada.
