American citizens are allowed to drive a two-ton death machine at age sixteen. They are allowed to go around the world and kill people professionally at age seventeen. At age eighteen, they can decide the future of the most powerful nation on Earth. But they are not allowed to enjoy a simple adult beverage for another three years after that. That, friends, is ridiculous.
One might ask: "why nineteen and not eighteen?" The reason is that many high school seniors will turn eighteen before graduating high school, and will be very likely to buy for minors.
Besides closing the logic gap observed above, lowering the minimum age will cut enforcement costs, since fewer people will need to be monitored. It will also enable the learning of responsible consumption habits at an earlier age.