Kids will take whatever they can to get high, booze, cigarettes, drugs, glue, etc.
Legalisation will increase tax revenue, provide legitimate business and establish much needed regulation of the illegal drugs industry.
Afghanistan will have a proper export to boot its economic status.
The police can target organised crime, uninsured drivers, robbers, etc and stop hassling the youth. They will only grow up disaffected, with a misplaced distrust of the police.