Users will lack safety advice and controlled product quality. Black market dealers sell to buyers of all ages, and they often sell many kinds of drugs. Whatever health risks there are from a drug, they will always increase if the drug is banned and in proportion to how hard the law is enforced. When such things happen, the actual outcome of what was a well-meaning policy leads us away from the goal of protecting society. People died or went blind from bad liquour.
Addicts are younger than ever.
In Japan where marijuana is treated no different from harder drugs many young people who experiment with drugs do not use less harmful cannabis as in more cannabis-tolerant countries but try far more dangerous solvents or amphetamines because these are more readily available. In the last decades, an alternative approach to dealing with the drugs problem has gained support in many countries. It takes into account the law of unintended consequences. If total eradication of currrently prohibited drugs is an unrealistic goal then we need to find a way to live with the availability of drugs, striving to minimize harm caused by both drug use itself and by the direct and indirect effects of our policies meant to discourage drug use. Harm reduction is not just a theory.