Background of the Issue Should the Drinking Age Be Lowered from 21 to a Younger Age?
All 50 US states have set their minimum drinking age to 21 although exceptions do exist on a state-by-state basis for consumption at home, under adult supervision, for medical necessity, and other reasons.
Proponents of lowering the minimum legal drinking age (MLDA) from 21 argue that it has not stopped teen drinking, and has instead pushed underage binge drinking into private and less controlled environments, leading to more health and life-endangering behavior by teens.
Opponents of lowering the MLDA argue that teens have not yet reached an age where they can handle alcohol responsibly, and thus are more likely to harm or even kill themselves and others by drinking prior to 21. They contend that traffic fatalities decreased when the MLDA increased.
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Bar graph of current, binge, and heavy alcohol use among persons aged 12 to 20, by gender in 2007. Source: Figure 3.7 in "Results from the 2007 National Survey on Drug Use and Health: National Findings," www.oas.samhsa.gov/NSDUH, Sep. 2008
The repeal of alcohol prohibition by the 21st Amendment on Dec. 5, 1933 allowed each state to set its own alcohol consumption laws. At that time, most states established the MLDA for alcohol at 21 years of age, although two states set an MLDA of 21 for men and 18 for women: Illinois (1933-1961) and Oklahoma (1933-1976). The 1976 US Supreme Court case Craig v. Boren ruled 7-2 that this age difference violated the Equal Protection Clause of the 14th Amendment. Following the July 1, 1971 passage of the 26th Amendment, which lowered the legal voting age from 21 to 18 years of age, 30 US states lowered their MLDA to 18, 19, or 20; by 1982, only 14 states still had an MLDA of 21.
Reports in the 1970's showing that teenage car accidents increased in states where the MLDA had been lowered from 21 prompted Congress to pass the National Minimum Drinking Age Act of 1984. 
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Bar graph showing 77% of Americans opposed to lowering the drinking age to 18 nationwide, polled July 12-15, 2007. Source: "Most Americans Oppose Lowering Legal Drinking Age to 18 Nationwide," www.gallup.com, July 27, 2007
Although the Act did not require a national MLDA of 21, it effectively mandated it by stipulating that some federal transportation funds would be withheld from states that failed to make 21 their minimum age for purchasing and publicly possessing alcohol. Since 1984, all states that had previously lowered their MLDA from 21 have all raised their MLDA back to 21. South Dakota and Wyoming were the last states to do so in 1988. 
Proportion of 83 countries' MLDAs (if any) from ages 14 to 21. See source for the list of countries included and their drinking ages. Source: Cognac, "Legal Drinking Age In Different Countries," www.cognac.com, May 22, 2009
While the MLDA is 21 in all 50 states, in 47 of 50 states age 18 is the "age of majority," which entails having the rights and responsibilities of adulthood.  Every state sets its own age of majority that often corresponds with the age at which one can vote, join the military, serve in jury duty, sign contracts, marry, apply for loans, make decisions regarding medical treatments, and be prosecuted as an adult. Alabama (age 19), Mississippi (21), and Nebraska (age 19) are three states that have an "age of majority" above 18, although certain rights such as the right to vote remain at 18 in these states. 
The discrepancy between the MLDA and the age of majority--and its many responsibilities and authorities--along with continued incidents of alcohol abuse reported on college campuses have fueled debate on whether or not setting the MLDA at 21 is fair, smart, and effective.