Wouldn't it still be illegal for a kid to try any drug, just as it's illegal for them to try alcohol now?
I know that kids that want to drink today, manage to get their hands on beer and such - the same is true for drugs - legal or not.
It could also be said that kids try drugs because they are illegal (but I don't buy that one, either).
Personally, I think the legality of a drug is way down on the checklist that might deter a kid from trying it - if it even is a consideration at all.
Fear of the effects, parental ramifications, peer pressure, are among the top of the list, IMHO.
Agreed on peer pressure, etc...
But by legalizing a drug, are you not in fact saying that a society does see a particular drug as being acceptable at a certain age?
And by making that assertion, in the mind of a teenager, you've effectively said, "You're just not adult enough. But soon you will be, and then you can use this." So in the teenager's mind, it's not fair that adults get to use a drug and they don't. Or it's intriguing to play and see what being an adult is like. There's just not the level of intimidation that's there for illegal drugs. Further, you're pitting a teen's natural rebellion to become an adult by creating an adult/non-adult substance v. simply leaving something out-of-bounds for an entire society, children and adults alike.