This Line of defense must be kept at 21 - no questions.
By lowering the limits you are pushing potential problems to even lower -pre-legal drinking age. If you want to tech your kid to drink responsibly - nothing prevents you to do it now on your own premises as long as it does not effect others - your neighbors - your visitors... And if you as a parent can't do it responsibly this way at your home - what will you be able to achieve by lowering drinking age - creating "responsible teens drinking parties?"
Just imagining a bunch of high school kids attending one of those and spilling out on the street makes me fill creepy. Kids these days barely can keep their boundaries of respect to authorities - forget parents, add a little buzz to it and you get yourself highly volatile explosives! Don't be crazy - teach your kids to respect the law and if you need to tech them responsible drinking - do it in privacy of your own family - do not try to hand over responsibility for that to our overstretched police and other institutions - this is crazy!