Yes, as I said the general illegal market will drop, but the only way for teens to buy it will still be illegally, so that part of the market will survive, like an illegal niche if you want. That is if they don't find ways to access the legal stuff anyway, as they do with alcohol in the US. For example, this report from Colorado, made after 5 years from the legalization there, points out that "[regarding teen consumption] we've seen almost statistically flatline use [...] for the past decade."