There are many reasons why people think the legal drinking age should be 18. While most of us agree the legal drinking age should at least be 19 so that high school seniors aren't drinking.
The reasons the drinking age should be lowered are:
The drinking age used to be 18.
Many other countries do not regulate the age of consumption or it is 18 elsewhere.
Because young people do not want to wait to be 21 they find ways to break the law and get a hold of alcohol anyway. If they only had to wait 1 year from high school, for example, they might just feel like waiting.
Many parents and other guardians allow their kids to drink at home because it is their philosophy that they would rather see their children drink supervised than do the inevitable unsupervised and to also teach safe drinking.