I hear this argument often but it doesn't account for the fact that drugs are already available in every school and all teenagers do not fall into a life of addiction. Even among drug users addiction is relatively rare. There were some interesting study on this topic a few years ago that found vast majority of all drug users never become addicted.

As to the cultural and economic impact, the negative side effects are offset by the more open and safer system that is created when drugs are legalized. That is to say, your total number of users may rise but the negative consequences of their use is greatly reduced. They no longer have to deal with smugglers to get their drugs, the drugs are more pure and less likely to make a user sick, et cerera all counter the new problems that a society may face.

One could argue from a moral stand point but morals are personal and no two people have the same moral code.

The question now becomes what should a society value more? Is the freedom of the individual more important than norms of a culture? From my perspective, individual freedom is of the greater importance. Further I firmly believe that freedom includes the right to do the wrong things. If a person thinks it is a good idea to set himself on fire, I will think he is foolish, I will tell him it is a bad idea but I will not use force to stop him from doing it.

I could't find the origional study but this article covers some of the same information.
http://www.thecleanslate.org/reality-check-most-dont-become-addicted/