Legal Drinking Age In The United States

What I know: From what I know you have to be twenty-one years of age to legally purchase or consume any type of alcohol in the United States. The person purchasing the alcohol must have a valid state ID or drivers license. I also know it is illegal for anyone to purchase alcohol and provide it to individuals under the required age of twenty-one. Almost all other countries have no or a lower drinking age. What I'm interested in knowing is why The United States legal drinking age is twenty-one but to enlist in the military you only have to be eighteen? Is there any chance of the legal drinking age ever being lowered? If the legal drinking age is ever lowered what would it be lowered to? My audience would be people under the legal drinking age or anyone with interest in lowering the drinking age.

confused

I wonder if I did this right?