Prohibiting this age group from drinking in bars, restaurants, and other licensed locations causes them to drink in unsupervised places such as fraternity houses or house parties where they may be more prone to binge drinking and other unsafe behavior. Also i can receive the rights and responsibilities of adulthood to vote, smoke cigarettes, serve on juries, get married, sign contracts, be prosecuted as adults, and join the military - which includes risking one's life.
Parents currently have to spend time worrying about whether or not their adult children are drinking before 21 which would be considered breaking the law. Lowering the drinking age to 18, would make it so parents no longer have to worry about this. Not to mention that you are already considered an adult at age 18.
Although I personally have no issues with the lowering of the drinking age, how in the world would it help parents? Parents are going to worry about their children no matter what age they are when they start drinking whether it is legally or not. Lowering of the drinking age helps no one except the government who will get more taxes and the 18 year old who wants to drink.