I'm not being sarcastic. I'm a libertarian (i.e. classical liberal). I think people should have freedom to do things that don't hurt others.
It's only legitimate to stop people drinking when there's some danger involved, e.g. drinking with driving. One can argue for other cases like drinking heavily with a "mean drunk" personality type, or drinking heavily without the experience/knowledge to be safe in that situation. Drinking with being aged 20 isn't inherently dangerous. And it's a bad (both inaccurate and ageist) proxy for the other things that actually matter.
