If the drinking age is lowered to 18, history is likely to repeat itself. In 1971, Congress ratified the 26th Amendment, lowering the voting age from 21 to 18. Afterwards, the public called for the drinking age to be lowered to be in line with the voting age, and the government responded. After it was lowered, there was a significant increase in alcohol-related car accidents. Congress demanded that the states raise the drinking age back to 21, or risk losing millions of dollars in highway construction.
Studies have shown that the human brain doesn't fully mature until around age 25. 21 is close to 25, but it isn't actually 25. In fact, there are rumors that there may be a bill proposed in Congress which would actually bump the drinking age up to 25. Also, it is definitely likely that the number of alcohol-related car crashes might increase because 18-year-olds aren't fully mature.