Title: Legal drinking age controversy in the United States
Content:  The legal drinking age varies from country to country. In the United States, the legal drinking age is currently 21. To curb excessive alcohol consumption by younger persons, instead of raising the drinking age other countries have raised the prices of alcohol beverages and have encouraged the general public to drink less. Setting a legal drinking age of 21 is designed to discourage reckless alcohol consumption by youth, limiting consumption to those who are more mature, who can be expected to make reasonable and wise decisions when it comes to drinking.
Title: National Minimum Drinking Age Act
Content:  Legislation concerning the legal minimum drinking age in the United States can be traced back to the days of Prohibition. In 1920, the 18th Amendment to the U.S. Constitution declared it illegal to manufacture, transport, or sell intoxicating liquors. This was repealed with the passing of the 21st Amendment in 1933, which was followed by the adoption of minimum legal drinking age policies in all states, with most states electing a minimum legal drinking age (MLDA) of 21. Between 1970 and 1975, 29 states lowered the MLDA from 21 to 18, 19, or 20. This was primarily due to the 
Title: National Minimum Drinking Age Act
Content:  states raised the minimum legal drinking age to 19 (and sometimes to 20 or 21). In 1984, the National Minimum Legal Drinking Act, written by Senator Frank Lautenberg (D-NJ) and influenced by Mothers Against Drunk Driving (MADD), required all states to enforce a minimum legal drinking age of 21 or else risk losing 10% of all federal highway construction funds. As the MLDA was still left to the discretion of the state, the act did not violate the 21st amendment which reserved the right to regulate alcohol for all responsibilities not specifically appointed to the federal government to the states. 
Title: Legal drinking age controversy in the United States
Content:  own drinking ages. Despite this, the Federal Government can assess a 10% penalty on highway funds for states which choose to set their drinking age lower than 21. Less than a year after the Supreme Court ruling, all 50 states of the United States officially made 21 the minimum legal drinking age. Motor vehicle accidents decreased after the legal drinking age was raised, but the MLDA-21 is not the only variable which can be identified as a reason for the declining accident rate. The shift in demographics, increased enforcement, increased seat belt use, safer cars, increased parental monitoring, and "designated driver" emphasis are also factors that are likely to have played a part in lowering the vehicle accident rate in the U.S.
Title: Mothers Against Drunk Driving
Content:  enacting of a 1984 federal law, the National Minimum Drinking Age Act, that introduced a federal penalty (a 5%—later raised to 10%—loss of federal highway dollars), for states that did not raise the minimum legal age for the purchase and possession of alcohol to 21. After the United States Supreme Court upheld the law in the 1987 case of South Dakota v. Dole, every state and the District of Columbia made the necessary adjustments by 1988 (but not the territories of Puerto Rico and Guam). However, in July 2010 Guam raised its drinking age to 21. In 1985, Lightner objected to the shifting focus of MADD, and left her 
Title: Legal drinking age controversy in the United States
Content:  Alcohol is the most commonly used and abused drug among youth in the United States, more than tobacco and illicit drugs. Although the purchase of alcohol by persons under the age of 21 is illegal, people aged 12–20 years old drink 11% of all alcohol consumed in the US. Among the 14 million adults aged 21 or older who were classified as having alcohol dependence or abuse in the past year, more than 13 million had started using alcohol before age 21. Since 1984, when the National Minimum Drinking Age Act made the minimum legal drinking age for every state in the nation 21, there has been a steady increase in prevalence of alcohol use, heavy use, and frequent use among underage drinkers as the age increases. Across all ages, the highest rates for alcohol abuse occur among persons 19 years old due to illegality of their behavior, and peak alcohol dependence is age 22. In 2013, more than two-thirds of American students had consumed alcohol by the end of high school; and more than half of 12th graders had been drunk at least once in their lifetime.
Title: Alcohol consumption by youth in the United States
Content:  Most states decided on the age 21 after prohibition, but the 26th amendment allowed individuals 18 and older to vote and serve in Vietnam, so several states lowered the drinking age as well. However, President Ronald Reagan, who was influenced by groups such as Mothers Against Drunk Driving (MADD) and Remove Intoxicated Drivers (RID), decided to federally enforce a 21+ drinking age by signing the National Minimum Drinking Age Act of 1984. He believed that doing so would decrease the number of accidents related to drunk driving, because he said that states with a drinking age of 21+ did not have as many drunk driving accidents. Reagan said that although he wished states would create their 
Title: Alcohol law
Content:  The legal age for buying and possessing (but not necessarily for drinking) has been 21 years in every state since shortly after the passage of the National Minimum Drinking Age Act in 1984, which tied federal highway funds to states' maintaining a minimum drinking age of 21. Despite a rekindled national debate in 2008 on the established drinking age (initiated by several university presidents), a Fairleigh Dickinson University PublicMind poll found in September 2008 that 76% of New Jerseyans supported leaving the legal drinking age at 21 years. No significant differences emerged when considering gender, political affiliation, or region. However, parents of younger children were more likely to support keeping the age at 21 (83%) than parents of college-age students (67%). Seventeen states (Arkansas, California, Connecticut, Florida, Kentucky, Maryland, Massachusetts, 
Title: Legal drinking age
Content:  from 18 to 19 years in response to a few studies demonstrating an association between the lowered drinking age and increases in alcohol-related harms to youth and young adults, including increases in motor vehicle accidents (MVAs) and alcohol intoxication among high school students. Following MLDA reductions in the US, research in several states provided persuasive evidence of sharp increases in rates of fatal and nonfatal MVAs appearing immediately after the implementation of lower drinking ages. These scientific findings galvanized public pressure on lawmakers to raise MLDAs and, in response, the federal government introduced the National Minimum Drinking Age Act of 1984, which imposed a reduction of highway funds for states if they did not increase their MLDA to 21 years. All states complied and implemented an MLDA of 21 years by 1988."
Title: Alcohol laws of New York
Content:  In response to the National Minimum Drinking Age Act in 1984, which reduced by up to 10% the federal highway funding of any state which did not have a minimum purchasing age of 21, the New York Legislature raised the drinking age from 19 to 21, effective December 1, 1984. (The drinking age had been 18 for many years before the first raise on December 4, 1982, to 19.) Persons under 21 are prohibited from purchasing alcohol or possessing alcohol with the intent to consume, unless the alcohol was given to that person by their parent or legal guardian. There is no law prohibiting persons under the age of 21 consuming alcohol that was given to them by their parent or legal guardian. Persons under 21 are prohibited from having a blood alcohol level of 0.02% or higher while driving.

Question: when did the legal drinking age changed to 21?
Answer: By mid-1988.
October 1986.