Drinking alcohol lightly has both risks and benefits. Older adults who drink a little bit (like a drink or two a day at most) tend to live longer than people who don't drink at all, and also live longer than people who drink way too much. This may be because alcohol (or byproducts of the manufacturing process) influences blood clotting, thereby reducing the risk of heart attacks. The risk associated with light alcohol drinking, of course, is that a person may start drinking more and lose control over the drinking behavior. New research suggests that this is more likely to happen to adolescents compared to adults.
There is another big added risk for teenagers drinking alcohol, and that is that the brain is still fine-tuning and developing until the mid 20's. Drinking while the brain is still forming is like riding a bicycle over the sidewalk when the cement has just been poured and is still drying, versus riding over it later after the cement has hardened. Just as you are likely to leave lasting impressions and ruin the sidewalk when you run over it while it is still fresh, so too you can disrupt the final wiring of your brain when you drink alcohol as a teenager.
Since raising the drinking age from 18 to 21, many thousands of lives have been saved through decreases in automobile crashes. In fact, one report showed that by 2002 more than 20,000 lives had been saved. (See http://www-nrd.nhtsa.dot.gov/Pubs/809860.PDF [PDF - 143 KB].) Also, we know that the earlier you start using drugs, including alcohol and cigarettes, the more likely you are to develop drug problems when you get older. There are likely many reasons for this, but you should know that the brain is still developing during the teen years, and drugs may affect that development.