This author asks, Why is the drinking age 21 in the U.S.?

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http://fff.org/explore-freedom/article/why-is-the-drinking-age-21/

This is a pretty reasonable question to ask.  

Certainly setting the drinking age at 21 doesn't stop young adults, including college students, below the age of 21 from drinking.  Some statistics:

Underage Drinking

Underage drinking is when anyone under the minimum legal drinking age of 21 drinks alcohol.

Underage drinking is a risk that attracts many developing adolescents and teens. When young people try alcohol, they often don’t realize the damaging effects drinking can have on their own lives, their families, and their communities.

Aside from being illegal, underage drinking is a widespread public health problem that poses many risks. 

Here’s why:

Underage drinking is widespread

• In 2009, about 10.4 million young people between ages 12 and 20 drank more than “just a few sips” of alcohol.
• As kids get older, they drink more.  By age 15, half of teens have had at least one drink. By age 18, more than 70% of teens have had at least one drink.

Young people drink a lot

• Young people drink less often than adults, but when they do drink, they drink more than adults.
• On average, young people have about 5 drinks on a single occasion, which can be considered binge drinking.

http://www.niaaa.nih.gov/alcohol-health/special-populations-co-occurring-disorders/underage-drinking

Underage Drinking

• During the past month (30 days), 26.4% of underage persons (ages 12-20) used alcohol, and binge drinking among the same age group was 17.4%.
  SAMHSA

• Alcohol use remains extremely widespread among today’s teenagers.  Nearly three quarters of students (72%) have consumed alcohol (more than just a few sips) by the end of high school, and more than a third (37%) have done so by eighth grade.
  NIDA

• Past-month alcohol use rates declined between 2002 and 2008 for those ages 12-13 (4.3% to 3.4%), 14 or 15 (16.6% to 13.1%), 16 or 17 (32.6% to 26.2%), and 18-20 (51.0% to 48.7%).
  SAMHSA
• Among race demographics, whites had the highest percentage of underage (ages 12-20) past-month alcohol use (30.4%). Asians had the lowest rate at 16.1%.SAMHSA
• In 2008, 56.2% of current underage drinkers (ages 12-20) reported that their last use of alcohol occurred in someone else’s home; 29.6% reported that it occurred in their own home.
  SAMHSA
• Among underage drinkers (ages 12-20), 30.8% paid for the alcohol the last time they drank – including 8.3% who purchased the alcohol themselves and 22.3% who gave money to someone else to purchase it. Among those who did not pay for the alcohol they drank, 37.4% got it from an unrelated person of legal drinking age; 21.1% received it from a parent, guardian, or other adult family member.
  SAMHSA

http://sadd.org/stats.htm

Now consider this:  The nations that have 21 for the legal drinking age are Indonesia, Kazakhstan, Oman, Pakistan, Palau, Sri Lanka... and the United States.  Only six other nations have 19 or 20 as the legal age.  All the rest of the world sets the age at 18 or below:
http://www2.potsdam.edu/hansondj/legaldrinkingage.html#.Uok3zSjFVT4

Are we and the Indonesians and Pakistanis and Sri Lankans right and the rest of the world wrong?  Or is this making of strange bedfellows a consequence of our weird history of Prohibition.   I mean, we can't pass an Equal Rights Amendment, but we were able to pass a Constitutional prohibition of alcohol... and we saw how that worked out.
Amazingly, some people want to change the legal age to 25; that'll work out well, I bet.

• Changing the Legal Drinking Age to 25.... (theadventuresofprincesssophia.wordpress.com)
• Teens Get MADD About Underage Drinking (detroit.cbslocal.com)
• Drinking Age Final Draft (arrodrigadita.wordpress.com)
• Repeal 21-only to start talk on drinking age (thegazette.com)
• Should the Minimum Legal Drinking Age Be Lowered to 18? (writemyessaycheap.wordpress.com)
• Study: 41 percent of 18-20 year olds in Alabama are 'binge drinkers' (yellowhammernews.com)
• Some Additions to the "Legal Drinking Age" Debate (ellygoesrawrr.wordpress.com)
• Supporting Evidence with Reason (eng101blogfall2013.wordpress.com)
• The Pros and Cons of Changing the Drinking Age (alcoholism.answers.com)