I haven't read through this thread, I really can't be bothered, so I'll just say three things about this and bugger off.
1. I work in retail on a checkout. The store where I work attracts a large number of people from all races, ages, creeds, sexual orientations and whatever else you can think of. Serving such a diverse range of people has taught me one thing: no one group of people has a monopoly on arseholes. I have great customers from all age and racial groups and bad customers from all groups. To say that all 18 year olds are irresponsible and stupid is painting with a very wide brush; many people from that age do suck but many are great, just as it is for every other age group.
To deny people from 18 to 21, or in some cases 16 to 21, the right to drink and make choices for themselves is punishing the good ones because of the bad ones.
2. At 18 you're legally responsible for your actions, yet you're not responsible enough to drink? If you're officially responsible enough for all of your actions, shouldn't you therefore have the rights to go with that responsibility?
3. Sheltering people from the world doesn't help them. Let people learn things for themselves, even if it's the hard way. Yes, they're going to make mistakes, but that's their prerogative and is necessary for their development. In fact, I would argue it's better to learn these things first hand rather than just be told in classroom where everyone is half asleep or looking down the teacher's blouse anyway.