Change the federal drinking age back to 18

Many years ago, the drinking age was changed to 21, and many 18-20 year olds have gotten in trouble with the law due to "underage drinking." However, at 18, people become legal citizens and can now, go to war, vote, and legally buy tobacco in some states. How can an 18 year old be allowed to go across the world to fight for our country but not be allowed to drink or buy a beer when they come home from an overseas tour? We the people would like the Obama administration to address this issue or change the federal drinking age back to 18.