Exactly. Will there be abuses at first? Sure. Everytime they
  lower the drinking age, there's problems at _first_. But give it
  enough time, and the excitement will settle down. They usually
  bring it right back up again whenever they've tried it in the
  past. Plus, I thought of this too: It's often hard to tell the
  difference between 18 and 21 yr olds at a glance.. But, it's
  usually easier to tell the difference between 16+18 yrs old. [I
  think, anyway and of course this is a huge generalization]
  Thinking as if I was a person whose job it was to serve alcohol
  (liquor store, bar, etc), it would be a lot easier to proof on
  sight. The reason I'm thinking this is: I think ppl do most of
  their drinking before it's legal, often because it's not legal
  yet and once it's legal it's like, "meh, whatever". But if it's
  legal (and once it stops being novel) at 18+, the "ooh, it's
  illegal factor" will drop in age and anybody whose job it is to
  play "age spotting" to keep ppl from breaking laws would likely
  have a much easier time of it. I know it's superficial - likely
  ridiculous. But I'm trying to think of it from a systems point
  of view: how it would affect day-to-day activities in a town or
  city and all of the people that are involved in the process of
  liquor usage and regulation. But ultimately, I agree. You're 18.
  You're supposed to be an adult and yet you're not yet. This
  extends to other things as well, such as minimum age to be
  President of the USA and such, but I guess changes have to start
  in different places for different reasons.