Well, you are not Jhhnn, so I will respond because, also, you have raised some good points and done so far more politely.
It seems where we disagree is on what is more likely to restrict access to cannabis from minors: legalization to adults or criminalization with significantly altered structure of law. Both of us agree that the current laws surrounding cannabis do a poor job and cost a lot of money and are disproportionate in punishment to people who would be better off left alone. Now, I don't sympathize with that last one as much, because people aren't breaking the law unknowingly and for any reason other than pleasure or profit.
Both of us recognize that, regardless of laws or regulations, we cannot remove cannabis from the hands of minors. The only questions are: which reduces use in this group the most, what are the economic costs, what are the human costs, and how do we weigh these variables with their impact on civil liberty? I think we disagree on the values of these variables and the equations used to weigh them, but I am satisfied that we agree on what they are and want to understand the values better. That's all I can hope for.
I don't think that my position that criminality lessens use can be assumed to be wrong. Speeding is illegal. It doesn't stop people from speeding, but I believe less people speed because of the law and its enforcement, and also that people who speed often go slower than they would still if it were not illegal. This is not an argument to support laws against speeding or if our speed limits are rational. It is merely an argument that laws have an effect.
I do not believe that the benefits of cannabis far outweigh the negative effects of it. I do not think that cannabis is good for society, though I believe it is good for some members of society. All I was saying is that this, to me, does not justify our liberty to decide on our own being infringed.
I do believe that protecting minors from use on the basis of their later risk of schizophrenia development, even if they discontinue use before adulthood, is reason enough for it to be criminal if laws are effective.
As far as prohibition as an example goes, it's a poor one. It happened nearly 100 years ago with a much different substance. And that was going from legal to illegal and not the other way around. We already did that with cannabis, and more recently than with alcohol to boot, so there's no reason to compare this to prohibition.
For the record, if cannabis were currently legal I would not be in favor of criminalizing it. I believe that action would have similar effects as you describe to prohibition of alcohol, which would be bad.
Nonetheless, cannabis is currently mostly illegal. My vote is for it to remain illegal but with a much different set of laws surrounding it.
I would be happy to hear data supporting the idea that legalization will lead to less use by minors. If I could become convinced that this is the case, then 100% I would be in support of the argument for legalization. I don't expect to become convinced, but you may make me more ambivalent than my current stance, which is fine by me since I'll probably sleep better because I believe legalization is inevitable anyway.
RE: whether this is contrary to my stance as a libertarian on principle, I say resoundingly no. Hopefully it is tenable to disagree with this aspect of the platform and yet still be a libertarian.
I think that the government absolutely has the authority, and in some cases, responsibility to govern civil liberties. Just as I believe that our government absolutely has the authority, and in some cases responsibility to wage war. Just because I support the authority/responsibility, doesn't mean I want us to declare war on Canada, and it also doesn't mean that I feel we have waged war appropriately and responsibly in the past.
Just this morning, I woke up in a house governed by many, many building codes. I am required to pay insurance on it. I took a shower and am required to restrict my use of water. I put on clothes; I'm required not to be naked in public. I got in my car, another piece of machinery governed by so many regulations. I am forced to pay insurance on it. I got on the road. So many laws to follow. I can't run a red light even though I can see for miles and no one is around because there is a camera there. I stopped at McDonald's for breakfast. Can't have trans fat. Wanted to bring some cheese as a snack. Can't have it if it ain't pasteurized, etc. etc.
My civil liberties are restricted in many ways and in many ways with far more impact than not being able to smoke marijuana. And in many of those ways, purely to protect me from harming myself rather than others.
I believe that the government has its way with us far too often, and I believe those regulations are often corrupt, ineffective, and place barriers not just on freedoms but also on ability to enter business, etc. Those are why I'm libertarian, but I'm also glad that the government has the right/responsibility to erect laws that might infringe my freedoms in some limited fashion because it helps society as a whole, and there's a lot of crap I don't want to have to worry about protecting myself from.