Why is the legal drinking age in the United States 21 and not 18?

I have asked this question many times and almost always received the same answer. "One is still a baby at 18, and besides not being very mature, many tend to be extremely irresponsible."

Personally, I feel as though this is not true, and that it is a stereotype and a cop-out. If 18 year olds are babies and irresponsible, why are they allowed to enter the armed services? How can someone be recruited into the service, fight for his/her country, risk his/her life, and defend his/her nation, but still be classified as immature and irresponsible? I sure don't know, but I am certainly seeking an answer.

Whatever happened to that saying, "Once you turn 18, you'll be an adult, and you can do whatever you want"? If you're now an adult and able to vote in the presidential elections, you should certainly be able to purchase alcoholic beverages. n