@Farmgirl25,
A couple of reasons:
Alcohol decreases teens' ability to pay attention.
The younger a person is when they begin drinking, the more likely they are to develop a problem with alcohol.
Each year, almost 2000 people under the age of 21 years died in car crashes in which underage drinking is involved. Alcohol is involved in nearly half of all violent deaths involving teens.
More than three times the number of eighth-grade girls who drink heavily said they have attempted suicide compared to girls in that grade who do not drink.
Teens that drink are more likely to engage in sexual activity, have unprotected sex, or have sex with a stranger.
Excess alcohol use can cause or mask other emotional problems, like anxiety or depression.
Drinking in excess can lead to the use of other drugs, like marijuana, cocaine, or heroin.
And a couple of links:
http://kidshealth.org/teen/drug_alcohol/alcohol/alcohol.html
http://www.medicinenet.com/alcohol_and_teens/article.htm