If you are considered an adult but are still regulated as a child then are you truly an adult? At 18 you are allowed to make decisions about yourself without parental consent, ranging from changing your body all the way to fighting and dying for your country. There is no reason an 18 year old shouldn't be able to make a judgment on being responsible enough to drink or not.
I do agree that military members should be able to drink at the age of 18 as long as it is on base and they do not leave once they began drinking. This is actually in effect at quite a few bases. It is not popular, but it is gaining in popularity. They are forced to mature and gain life experience and attend classes regarding alcohol much faster than the majority of the U.S. pop and therefore are able to understand, plan and make much safer decisions regarding the consumption of alcohol.
HOWEVER, the majority are the people not in the service or who went through the same courses to understand alcohol. How can you say that these people who just got off of their parents teat and graduated high school are ready to risk their life and the lives of those around them with alcohol. They are not ready. They have barely begun in life and do not have the self restraint, understanding, maturity to handle it. I'm not saying a 21 year old does either, but at least he has a few years to grow accustomed to the life and realize they are accountable for themselves now. Allowing under 21 year olds to drink without limits will create more problems than what we currently have.