Every country has its own laws about who can and cannot drink. Often, laws are not enforced strictly, or are set aside if a parent or guardian is present. In some countries a parent can buy glasses of wine for the teenagers at the table in a restaurant, while this is forbidden in other countries.
Stop by the Wine Forum to discuss a particular country if you are planning a trip - get the full details from people who have been there!
Country Age Restrictions
Australia 18
Belgium 16
Canada 18
France 18
Germany beer/wine 16, rest 18
Ireland 18
Italy none
Japan 20 legally, 18 practically
Mexico 18
New Zealand 18 w adult / 21 alone
United Kingdom 5-drink,16-buy
United States 21
http://wine.about.com/library/encyc/bl_drinkage.htm