Well, any number we set would be ultimately arbitrary. For example, suppose I have a liquor store. The law is, "You can't sell alcohol to anyone unless they are mature enough to drink responsibly." That would be non-arbitrary, but how exactly could I follow that? I can't really judge the maturity of every person who comes into my store or anything. We need to set some kind of hard number to make any kind of restriction enforceable. Age seems like the only viable option. Ideally, the number would be based on some kind of science backed data. As for why it's 18, I imagine that came to be because it roughly coincides with the end of puberty for most people.
Yeah, I agree. There is not much of a different, alternative approach here. But what about differences depending on countries? I guess (without reading up on any of it) that the US allows alcohol consumption later than Austria could have something to do with the countrys history of prohibition.
And prohibition is another odd matter. Alcohol is more dangerous than what people in this particular thread like to consume. Yet alcohol is available in most places while weed is illegal. Cocaine was legal for a time as I understand it. That is cazy to me. Than there is nicotine and caffeine. Even stuff like sugar (perhaps my most severe addiction) is super unhealthy but is not really regulated in any way. - Perhaps in a hundred years time things are flipped and you could smoke as much cannabis as you like, but when the munchies hit, you have to go to your secret donut stash. :)
In the US, states had different drinking ages. But, this led to situations where it encouraged people to drive to another state where they can get drunk, and then drive back. They had to get every state to use the same age, which they did by withholding funding for highway related things for anyone who didn't go along. I think it was politically easier to get states to raise the drinking age than to lower it. The only people who would really object to raising it were 18-20 year olds, who wielded little political power. If the drinking age had been universally 18, it would probably be 18 still.
I agree with you on weed, although it is now legal where I am, and I'm so happy about that. I think the difference is, or at least should be, how much your behavior harms others. The thinking would go that while sugar is unhealthy, the brunt of the effects are felt by the individual (even if there are indirect costs that are spread to society), so individuals should have the right to decide. On the other hand, something like alcohol or Cocaine tend to have more severe effects on others either through the actions people under the influence take or in the difficulties in dealing with someone who is addicted, which I would argue is more than the problems in dealing with a fat person. So, regulation makes more sense in those circumstances.