Fallacy? 18 should be the drinking age because it's the age of adulthood in the U.S.

Something about this strikes me as fallacious... maybe it's a false cause and effect?

"The drinking age should be lowered to 18. 18 is the age of adulthood in the United States, and adults should have the right to make their own decisions about alcohol consumption."

I wouldn't say it is a fallacy; but it is not good reasoning either. "Adult" is simply a label we give to people of a arbitrary age. A rational thinker would consider the pros and cons of allowing 18-year-olds to drink legally, which includes looking at historical data, statistics, other countries and cultures, etc., then concluding that they should or should not be allowed to drink legally because of far better reasons than "we call them adults."

Conversely, one can counter "21 should be the legal age of adulthood because it's the age one is allowed to drink legally in the U.S." Now, this "neutralizes" both arguments and forces one to consider the more important factors mentioned above.

while it is definitely an argument, this is more of a valid opinion than an error in reasoning.

not a fallacy! just some argument that has to be weighed for its merits.