In an article entitled "Getting Stupid" by Bernice Wuethrich which appeared in a 2001 edition of Discovery Magazine, the author discusses this very topic. She cites a study by Michael de Bellis from the University of Pittsburgh Medical Center which involved MRI comparisons of the hippocampi portion of the brain of young people 14 to 21 years old who abused alcohol to those who did not. De Bellis found that the hippocampi (the part of the brain responsible for memory and learning) was roughly 10 percent smaller in the brains of alcohol abusers than it was in the brains of non-drinking young people.
Wuethrich discusses that alcohol's effect on young people who abuse alcohol is much more drastic than it would be on mature adults because the brain is still growing in teens, and alcohol has the potential to cease some brain growth. In an adult, the brain is fully grown (although it might not seem that way sometimes!), and so while he or she might drink and lose brain cells, at least the adult's brain presumably had the opportunity to reach maturity.