Drinking age in the US was raised to 21 because a pressure group convinced the US government to punish states that had a lower age!

This might seem strange to our readers from outside the United States. Although all around the US, the drinking age is strictly set at 21, it's actually the individual states that determine the drinking age for their own state. So how is it that we effectively have a 21-year-old drinking age all around the US?  

Cari Lightner, a 13 year old, was killed after a drunk driver hit her in 1980. Her mother, Candy, became a strong advocate against drunk driving and started the organization Mothers Against Drunk Driving. In 1983, a television movie about Lightner was released that brought much attention to MADD. 

In the early 80s, the group caught the attention of Congress and a New Jersey Senator who didn’t like that Jersey kids could run over to New York and get alcohol. The drinking age was 21 in Jersey, but only 18 in New York. It totally undermined Jersey law. 

By 1984, MADD was able to get a federal law passed that penalized states who didn’t raise their drinking age to 21 with a 5% and then a 10% loss of federal highway dollars. The law was brought to the Supreme Court who ruled for the law in 1987 in the case of South Dakota vs. Dole. After that, every state, including the District of Columbia, raised their legal drinking age to 21.

(Source)