Read up on US prohibition and how it funded the Mafia. It just changes the form of the societal disease.
The answer to addiction is having support and care on place for those that fall to it so society helps pick them up again. You can't stop the abuse of substances unless you fix why people are crawling into a hole to avoid the world. Lack of mental health is a disease of society as well as the individual.
What I meant was that "a ban from a certain age" is a total ban eventually. Black market will grow as the ban becomes more and more complete.