In most places it's easier for a 16yo to score "harder" drugs than beer and cigarettes. The law is what it is, but where there's a will there's a way, and the more you tell young people not to do something the more likely they are to do it out of spite and the desire to rebel against authority. If the powers that be understood this, they'd see they're the main driving force in young people abusing drugs.
No problem with placing age restrictions on things, so long as they're enforced consistently and coherently.
On a side note, the "just say no" horseshit from the Reagan era was the drug abatement equivalent of absintence only sex-ed. If you have any doubts as to the effectiveness of such tactics, ask Bristol Palin how effective absintence only propoganda was at guiding her behavior.