The thing is that lowering the drinking age doesn't dramatically raise the availability of alcohol for 18-year-olds. I know that when I was underage, I had no problem getting booze. And I work with teenagers now: they still don't have problems getting booze. What the higher drinking age does do is drive the drinking underground. So 18-year-olds have to drive more so that they can drink at bush parties. Or when a friend gets alcohol poisoning, they are afraid to get help.

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.