The way drinking laws are enforced effectively means you can’t drink in public till age 21, but are free to do so in private households. I can see why this is desirable. Kids start drinking excessively usually around freshman year of college (age 18), and tone it down by senior year (age 21). This confines the wild behavior of young drunks to private locations, where they won’t disturb the general public with vomiting, undressing, shouting, etc. By the time they’re 21, they have learned to hold their alcohol and have acquired some real-life responsibility. They can be trusted to drink socially in a public place.
The best counterargument, in my opinion, is that 18 year olds can vote and must register for selective service. If we can trust them with the future of the country, and expect them to die for us in armed conflicts, surely they deserve to be treated like adults for drinking purposes. Our parents’ generation made that argument–and that’s why, briefly, they could drink at age 18. If I recall correctly, the armed services on military property or out-of-country are still permitted to drink at age 18.
Anyway, the cooking extract alcohol problem resembles the Sudafed meth problem. I don’t think there’s an easy way to keep people from converting ordinary substances to drugs and alcohol.