It certainly is unhealthy for an 18 y/o to consume alcohol, and your "no more unhealthy than a 21 y/o" comment doesn't change that. Alcohol causes stomach cancer (really sucks), liver cancer (that sucks too), mouth and throat cancer (totally sucks), breast cancer (also sucks). Alcohol deforms the unborn. Alcohol indirectly causes car crashes, pregnancy, STDs, drowning, and all sorts of criminal charges. If homeschooled students disapprove, good!
The equal LSD availability, combined with reduced alcohol availability, suggests that "turn around and have a beer" isn't happening in these homes. LSD use is rare everywhere, but alcohol is only rare in the homeschooling homes.
Your fear of alcohol is incredibly irrational. There is absolutely no evidence that moderate alcohol consumption causes any of those things.
All of the cancer risks are linked to heavy drinking (usually 3+ drinks a day over a prolonged period of time), and there's absolutely no evidence that occasional moderate alcohol consumption poses a health risk greater than any number of other extremely low risk activities. Certainly not greater than a pack a day of cigarettes. (Furthermore, some studies have indicated that moderate alcohol consumption can even have positive health benefits, including decreased risks for some cancers.)
Similarly, alcohol in moderation causes none of the "indirect" things you mention.
You are free to assert an arbitrary moral superiority for abstention, but don't pretend like you have a rational basis for your opinions on the effect of moderate alcohol consumption. Pretending like all use is unhealthy is just as ignorant as the opposite extreme.
How do you know that your consumption will be moderate?
Some people lose control of their consumption. How do you justify any confidence that you won't be one of these people?
It's not as if the typical alcoholic just decided one day to be an alcoholic. For most it just... sort of happened. That could be you. Maybe not, perhaps probably not, but why would you take that risk? For little gain, you risk throwing away your life.
By consuming alcohol in moderation in the presence of a support network that will hold you accountable to that moderation. If people are not going to abstain for life -- and most won't! -- then learning how your body reacts to substances and how to moderate is an important life skill.
Tons and tons of people manage to drink without going am addiction.
And regarding risks, the same could be said for lots of things that are sometimes psychologically addictive -- internet, video games, shopping, etc.
The equal LSD availability, combined with reduced alcohol availability, suggests that "turn around and have a beer" isn't happening in these homes. LSD use is rare everywhere, but alcohol is only rare in the homeschooling homes.