answers for questions.

1. 2. How does alcohol effect the body? And why children are not allowed to drink?

Alcohol has many types and the one used in drinks is ethanol.
When ethanol is taken in the liver the body produces a enzyme to break it down.
This, ironically is more a strong poison than ethanol and causes hangovers and possibly cancer.
Alcohol itself stuns the brain and iternal organs and eventually stops the lungs and heart in short term.
Underage children are simply too weak to endure such a poison and it runs in the body quickly than adults. Adults take more than 15 or 20 years to even make a sign of the effects, but children are effected in less than a few months.

http://www.simplehealthguide.com/facts-about-alcohol-poisoning/
http://www.bowling-no9.com/yoi/mame01.php
http://www.asahibeer.co.jp/csr/tekisei/guidebook/risk_drinking/minor_drinking/

3. Why some groups intend to lower the legal age of drinking?
4. Why most people argue with that?

NGO's like Choose Responsibility wants to make alcohol a more close object so underage children wont be tempted to take them. They intend to do it in a more steady manner, like giving out education and making students learn about the dangers of alcohol before introducing them to the true thing.
Of course many people dissagree. Possibly because it could mean the return of alcoholic advertisement and a creation of a endless loop of lowering the age.