I think that if you're old enough to fight in a war you're old enough to buy a drink but college kids are clearly not off fighting a war.
Don't forget voting! We wouldn't want those 18 year olds to be the only people too sober to be duped at the ballot.
On a more constructive note: In Germany, the legal limit for drinking in public is 16 for "soft" drinks like wine and beer, 18 for everything else. When drinking in private, age limit varies from family to family, but is generally lower. Most of my friends got their first "official" drink served at their confirmation, or whatever coming-of-age ritual their religion provided.
I never heard of police raids for underage drinking in private households in Germany. The first one I witnessed happened when I was 18, and a 17 year old friend in Ashland, OR took me out for a party. I remember being shocked about the intrusion into the privacy of a household on such a trivial matter.
The consequences of our lower age limit? I don't have hard numbers, but by my own, subjective observations, German teenagers are much less immature about alcohol than Amerian teenagers. American teenagers get much more turned on, and then screwed up, by this whole forbidden-fruit thing.