A cursory glance at the history of youths tells anyone that it won't
And in this case you have a handful of companies that are responsible so it will be much easier to enforce vs trying to prevent every small shop from selling stuff illegally.
If you take that time period out, there is no decrease from the start of the war on drugs till the first iPhone in youth use of drugs.
Lastly, you forgot all these big companies started off as small, hard to regulate websites, which will be where kids flock to avoid 'teen accounts' and 'bans' and everything else