It's not illegal to allow your child to drink at home in America, is it?
Yes, it actually is against the law. It's just that nobody monitors what you do in your own home. If any law enforcement authority could ever find proof that you were providing alcohol to an underage minor, you would have some serious legal, or actually criminal problems to deal with.
Actually there is no federal law proscribing that, nor is there a federal law against drinking under 21. All of those laws are under state jurisdiction. The states themselves have to make those laws. Which is why mperez7372 mentioned that it was legal in New Orleans until a few years ago. Wisconsin also recently considered dropping their drinking age to 18; I believe that law failed, but they do allow 18-year olds to drink at home under parental supervision (and I believe it has to be their own parents, not their friend's). The reason why most people have this misconception is that the federal government has usually included a 21 drinking limit attached to their highway funding, meaning if a state chooses to have a drinking age lower than 21, they don't get the funds from the federal government. Which is why in some states this is a big debate... Can they get enough tax money generated from sale to those in the 18-20 year old range to compensate for the loss in money from the federal government.
I personally think that their should be a single age of majority. After that age, you can buy tobacco, view pornography, buy alcohol, make legal-binding agreements, go to war, drive a car, consent to sex, etc. As it is, we seem to have many different ages of majority, some of which seem backwards. In some states, the age of consent is 16 (or even younger) yet some movies are
Restricted for those under 18 (let alone
No
Children
17 and under or more hard-core pornography). Tobacco is also set to 18, yet alcohol is set to 21. Binding-contracts are set to 18 yet most rental agencies won't allow you to sign a rental agreement unless you are at least 25. And probably the most adult thing -- to sacrifice your life for your country -- is set to 18.
EDIT: Why is it that "pornography" is ok to write in the forums, but "p*rn" gets *beep*'ed?! (And I can't even substitute a zero?!)