A specific number has some advantages because it's less amorphous than a term like “vulnerable” or relying on some other qualitative factors. But it appears to be completely arbitrary in terms of an age. There is nothing of great import about the age of 16, except I suppose that historically you get your driver's licence at 16. I don't think you can do that anymore anyway.
This is throughout the Criminal Code. We see the age of 14, we see the age of 16, and we see the age when you move from the YCJA to adult sentences. I don't know if there are some evidence-based reasons why 16 is the age, or if there is some problem in the current case law—I didn't see any—that has that age of 16, or if it is just mirroring what is in the Criminal Code already.