I am trying to give you the german view on the american drinking age. I stayed 6 months in the states as an exchange student and visited the USA on a couple of other occasions.
German drinking laws:
The legal age to drink beer/wine/champagne is 16 and with 18 you are allowed to drink everything else.
You have to be 18 (with some exceptions) to drive and you have to go to a drivers school for theory classes and drivers lessons. The drivers license total cost is 1.5k+$. In the first year after you get your drivers license you are not allowed to be under the influence of alcohol while driving. Nobody wants to lose their 1.5k or their parents money for driving drunk.
I think driving under the influence is the biggest problem if you lower the drinking age in the states. The distance between houses is longer and most areas dont have a good public transportation system.
The other problem is that if you lower the drinking age to 18 is that freshmen can get their alcohol from seniors.
One solution might be to lower the drinking age for beer/wine to 18 for college students and for 18 year olds that are with a guardian.