Why a "legal age" for alcohol?

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Being that most of the industrialized European nations have the legal drinking age set at thirteen or fourteen, why do you suppose America feels that it should be twenty-one? I spent some considerable time overseas before I became an adult, and I didn't really witness nearly as many alcohol abuse problems amongst teenagers or young adults. Is it based on their upbringing or the fact that they readily have access to alcohol earlier in life (i.e, we always want to do what we arent supposed to).