Why the US drinking age is 21

 mentalfloss - In short, we ended up with a national minimum age of 21 because of the National Minimum Drinking Age Act of 1984. This law basically told states that they had to enact a minimum drinking age of 21 or lose up to ten percent of their federal highway funding. Since that's serious coin, the states jumped into line fairly quickly. Interestingly, this law doesn't prohibit drinking per se; it merely cajoles states to outlaw purchase and public possession by people under 21. Exceptions include possession (and presumably drinking) for religious practices, while in the company of parents, spouses, or guardians who are over 21, medical uses, and during the course of legal employment.

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