Nowadays almost of teenagers drink alcohol before they are 18. The younh people today want to experiment too much. They don't have enought fun at parties, discos, etc. If they don't have drugs or alcohol.
Why? possibly with this age of technology, where all our free time is with computers, video games and mobiles, we have no imagination. We don't know how to have fun. Perhaps all the extreme video games and films we watch we think normal life is boring.
Also we start to do things like go to discos too young and too often it is nothing special- so to make it special... Alcohol.
Alcohol and drug abuse before the age of 21 results in one part of your brain not developing - therefore your intelligence is also affected.
In conclusion I think that drinking alcohol isn't bad if you are old enought. But If someone drinks so much and their body gets used to it, then alcohol is destructive. Everything in moderations acceptable, but anything in excess is damaging.