One could just as easily say, "You can be 17 years and 364 days old and it is still illegal for you to star in a porn movie. In fact, if you did, you were breaking the law and so was anybody else who was making the movie with you. Yet the very next day, on your 18th birthday, you can now legally walk into a bedroom and have all the sex the camera can capture. What sense does that make?"
A society that proscribes children engaging in certain activities is compelled to draw an arbitrary line. When the line is drawn, there will be those on one side of the line who perhaps should be able to engage in the activity, but can't, and those on the other side of the line who should not be able to engage in the activity, but can. There are 12-year-olds who could responsibly drink wine at the dinner table, and 30-, 40-, 50-, and 60-year-olds who should never touch the stuff. A society of virtuous people would not need a government drawing the line, but if we were ever a society of virtuous people, we have long since lost that distinction.
Push the drinking age back to 18, and around 2,000 people annually will die in teen DUI accidents who are not dying now, based on how many died before the age was raised to 21. OTOH, a few hundred million dollars will not be spent on underage-drinking enforcement that is being spent now. If we wanted to, and were willing to use whatever means were necessary, we could use that money to essentially rid Africa of malaria, or rid India of cholera, and save many more than 2,000 lives--but that would require first the willingness to say that lives in Africa and India are as worthwhile as those in the US, second the willingness to allow older teens to live, or die, with the consequences of their own, sometimes immature choices, and third the ensuring that the money saved from the non-enforcement of "21" would actually go into effective programs to save those who are in danger of malaria or cholera. Those are three big "if"s, and if a society is not virtuous enough to handle individual choice in alcohol, it certainly would not be virtuous enough to handle the three "if"s.
As with just about anything (i.e. riding a bike, driving a car), you need to get your "training wheels" and "driving experience" before becoming a competent and responsible bicyclist or motorist.
Having a hard rule of "21" does nothing to prepare one for drinking responsibly. You could argue that it is the parents responsibility to train their children on responsible drinking in the home but they would be breaking the law. In fact, parents have gone to jail because they allowed under age drinking in their own homes.
So now we have this situation where turning 21 is a rite of passage where the individual goes off with a group of his friends to "tie one on." That is no way to be introduced to drinking.
We can’t assume the change in drinking age was the sole cause of reductions in DUI. There was a major cultural change during that same era in regards to DUI, in the 70s and early 80s there was an assumption of drinking and driving, drinking while driving even. I had neighbors that judged driving trips by 6 packs, and nobody thought it was a big deal, it was just how things went. We don’t think that way anymore, plus there’s much more draconian DUI laws to make sure even the people that want to think that way don’t. And vehicle technology has improved dramatically making crashes safer. I doubt there will be any increase in DUI fatalities with a lower drinking age, the drinking age really doesn’t prevent drinking by the young, it just moves it to darker corners of the world.
The teen DUI lives you think you would "save" by deferring the drinking age from 18 to 21 are just going to be snuffed out at age 22 rather than 19.
Make the drinking age 25 and those who can't handle it would die at 26.