Most of the good points WRT this topic have been made, but there are some left:
By saying that "all drugs are bad," you are setting yourself up to be discredited. The moment they try one that doesn't instantly turn them into a pathetic addict, you've lost credibility.
There are some drugs that can seriously screw you the first time you try them, and there are others that won't hurt you even with repeated use. It should be treated more like sex and STDs, where you just make people aware of the risks and how to still engage in certain activities (be they premarital sex or drug use) safely.
Also, legalization would standardize it. People will continue using drugs whether they are legal or not, as demonstrated by the Prohibition Era. Buying drugs now is risky; you can't be sure of what you're getting or how potent it is. If you were to buy it like alcohol, cigarettes, or over-the-counter medication, you could be sure of the contents and strength.
While alcohol is frequently obtained by persons under eighteen, it is discouraged and if not difficult, not easy. The same goes for cigarettes. If we did that with drugs which are currently illegal, we might be able to shift the age-group for users up a bit.