I first want to say that you are correct. The Parliament of Canada can go beyond the Carter decision, which adopts the principle that physician-assisted dying is accessible to an adult. Of course, that raises the question on who is an adult and at what age adulthood is reached. Nevertheless, Parliament can go beyond that decision and look to provide physician-assisted dying to minors who are capable of making decisions. For many people, the big question has nothing to do with age, but rather with decision-making capacity.
Parliament has the option to go further than the Carter decision. However, the Criminal Code of Canada currently sets that age at 18 years. In the provinces, the age of majority is 18 or 19, depending on the province. So it could be possible for Parliament to determine at what age someone is an adult and to apply physician-assisted dying only to adults, but it could also be possible for Parliament to open up access to physician-assisted dying even further by making it available to minors who possess, and I repeat, a decision-making capacity, but who are not adults, strictly speaking.