Most parents will agree with that, unless their kids are between the age of 18 to 21. People always seem to know best for everyone else, especially everyone else's kids.
That is all true; but, I guess there are more people who vote over the age of 21, than under. So in order to change laws, there must be enough interest in having the law changed to 18 for alcohol consumption. I think it's a good thing that it is at 21 years, because the number of alcohol related deaths in that age bracket have significately decreased since the age has been raised.