If you think you were as responsible at 18 as you are now, then you're still very irresponsible...
Also, I would say that 80+% of people have their first drink before they legally can. This means that their parents usually have no clue they are drinking, and so they are less able to give their children guidance.
There are tons of laws that I disagree with in this country but this isn't one of them. In fact, I think penalties for selling to under age kids should be much more harsh than they are now.
I do think that the negative effects that driving drinking underground are greater than the effects that increasing drinking would have.
But actually, I have. A good friend's wife was killed by a drunk driver as she jogged on the sidewalk. The guy was way over 21, and absolutely blitzed. It was a repeat offense. Another very good childhood friend of mine had to be pried from his car by the "jaws of life" after getting slammed by a drunk-driven pickup truck on the wrong side of the freeway. Fortunately, he lived and did experience what I believe was a complete recovery.
That is what MADD was originally formed to combat, and it was (and I mean this without any irony) a righteous cause.
But now, we're at the point where people will be arrested, thrown in jail, and carry a criminal record for driving at BAC levels that have a lower incidence of accidents than driving while chatting on a cell phone ($50 fine in California as of last month). To me, this is clearly the neo-prohibitionists at work. It's wildly out of proportion.
I've also been stopped (in Palo Alto) by cops who saw me walk out of a restaurant/bar and get into my car. They pulled me over because - and I swear I am not making this up - "your idle was running loud... have you been drinking this evening, sir?" Not a single traffic violation, no speeding, no running a stop sign, nothing. They pulled me over because my "idle was running loud" (my car was an old peugeot, I was a poor recent college grad, so yeah, the idle did run a bit loud, though I wasn't aware this gives the police the right to pull me over and make me be their trained monkey for fifteen minutes).
They put me through a bunch of tests (stand on this leg, stand on that leg, look in the sky and count backwards) and another cop car showed up as they gave me a breathtest. Oh, what do you know, the guy with long hair who drives a peugeot isn't drunk. Well, so long sir, be safe tonight.
I was on a date - it's really fun to go through this while your date is watching from the passenger seat.
Considering my experience with drunk drivers, it's pretty remarkable that the police and Madd have managed to turn me into someone who is instantly suspicious of the motives of people who claim to be combatting "drunk driving". I see it as a way to pursue prohibition, and to erode a citizen's protection from random search of their property and person.