"Recent research shows that the brain continues to develop well beyond childhood — and throughout adolescence. This research raises concerns that underage drinking may affect short-term and long-term cognitive functioning, and may change the brain in ways that can lead to future alcohol dependence." – Former Acting Surgeon General Kenneth P. Moritsugu, M.D., M.P.H.

The brain is still developing in to the early 20s and alcohol use impedes that important progress, possibly irreparably. 
We must work to make sure that alcohol can not get into the hands of youth.

Adolescent brain development
Research has shown that the brain continues to develop into the early twenties. (Kuhn, Swartzwelder, and Wilson, 1998; White, 2001; Giedd, et al, 1999; Giedd, 2004) The pre-frontal cortex, the part that controls reasoning and cognitive ability takes the longest to mature. This is why drinking, especially heavy drinking, before the brain finishes development affects memory and damages this pre-frontal cortex regions (Crews et al. 2000; Spear and Varlinskaya 2005; White and Swartzwelder 2005). Since this region is responsible for ability to learn complex tasks, control impulses and organizing, this is a significant loss. And, while the research on this is still in its early phases, there are some studies that indicate that this damage may be permanent. (Brown and Tapert, 2004)

While many of these studies are done on animals, the impacts are seen in adolescent humans as well. The hippocampus, which is responsible for forming new memories, was noticeably smaller in youth who abuse alcohol than in their nondrinking peers (De Bellis et al., 2000). Additionally, studies show that alcohol use in adolescence decreased ability in planning and executive functioning, memory, spatial operations and attention – all of which are important to academic performance and future functioning (Giancola and Mezzich, 2000; Brown et al., 2000; Tapert and Brown, 1999; Tapert et al., 2001).

Teen alcohol use and the brain 
Teen alcohol use only exacerbates the problem because of the effect it has on the developing brain. While alcohol acts as a sedative to adults, teens show more sensitivity to the stimulating effects of alcohol (White et al. 2003). Therefore, teens are more likely to drink past the points where adults would end up passing out and end up engaging in activities such as driving even though they are too impaired to do so (Hingson and Winter, 2003).

Studies

• Brown, S.A., and SF Tapert. "Health consequences of adolescent alcohol involvement." In National Research Council and Institute of Medicine, Reducing underage drinking: A collective responsibility, background papers. [CD-ROM]. Committee on Developing a Strategy to Reduce and Prevent Underage Drinking, Division of Behavioral and Social Sciences and Education. Washington, DC: The National Academies Press, 2004.
• Brown, S.A., et al. "Neurocognitive functioning of adolescents: Effects of protracted alcohol use." Alcoholism: Clinical and Experimental Research 24 (2) (2000): 164-71.
• Crews, F.T.; et al. "Binge ethanol consumption causes differential brain damage in young adolescent rats compared with adult rats." Alcoholism: Clinical and Experimental Research24 (2000): 1712–1723.
• De Bellis, M.D., et al. "Hippocampal volume in adolescent-onset alcohol use disorders." American Journal of Psychiatry 157(17) (2000): 737-744.
• Giedd, Jay N. et al. " Brain Development During Childhood and Adolescence: A Longitudinal MRI Study." Nature Neuroscience 2, no. 10, October 1999. 
• Giedd, Jay N. “Structural Magnetic Resonance Imaging of the Adolescent Brain.” Annals of N.Y. Academy of Sciences 1021 (2004): 77–85. http://www.annalsnyas.org/cgi/content/abstract/1021/1/77
• Giancola, P.R., and AC Mezzich. "Neuropsychological deficits in female adolescents with a substance use disorder: Better accounted for by conduct disorder?" Journal of Studies on Alcohol 61(6) (2000): 809-817.
• Hingson, Ralph and M. Winter. "Epidemiology and consequences of drinking and driving." Alcohol Research & Health27(1) (2003): 63–78.
• Kuhn, Cynthia, Scott Swartzwelder, and Wilkie Wilson.  Buzzed: The Straight Facts about the Most Used and Abused Drugs from Alcohol to Ecstasy.  New York, NY: W.W. Norton, 1998. (not yet online)
• Spear, L.P., and Varlinskaya, E.I. "Adolescence: Alcohol sensitivity, tolerance, and intake". In: Galanter, M., ed. Recent Developments in Alcoholism, Vol. 17: Alcohol Problems in Adolescents and Young Adults: Epidemiology, Neurobiology, Prevention, Treatment. New York: Springer, 2005: 143–159.
• Tapert, S.F. and SA Brown. "Neuropsychological correlates of adolescent substance abuse: Four-year outcomes." Journal of the International Neuropsychological Society5 (1999): 481-493.
• Tapert, S.F., et al. "MRI measurement of brain function in alcohol use disordered adolescents." Alcoholism: Clinical and Experimental Research,25 (2001): 80A.
• White, Aaron M. "Substance Use and Adolescent Brain Development: An Overview of Recent Findings with a Focus on Alcohol." Durham, NC: Duke University Medical Center, 2001. 
• White, Aaron M. “Substance Use and Adolescent Brain Development: An Overview of Recent Findings with a Focus on Alcohol.” Youth Studies Australia, 22 (2003), 39-45.
• White, Aaron M., and Scott Swartzwelder. "Age-related effects of alcohol on memory and memory-related brain function in adolescents and adults." In: Galanter, M., ed. Recent Developments Alcoholism, Vol. 17: Alcohol Problems in Adolescents and Young Adults: Epidemiology, Neurobiology, Prevention, Treatment. New York: Springer (2005): 161–176.