So businesses do not openly target them with marketing and sell to them. It doesn't stop anyone from making their own or selling to friends.
Alcohol is comparatively difficult, costly and obvious to make while any idiot can grow a cannabis plant from seeds.
I knew kids that grew it in their very early teens and that was before the days of the internet. Today's kids can order high grade seeds from overseas and get expert growing advice online.
Then why have commercial advertising or sale of pot at all in Colorado? Let the users produce their own and anything more than personal stash is actionable...