Different countries have different legal drinking ages.
In England & Wales the age differs on circumstance;
18 to buy, consume, possess alcohol in public
16 to consume beer, wine or cider on licensed premises with meal
16 to buy liqueur chocolates
5 to consume alcohol at private premises
0 under medical supervision
The USA is wonderful, isn't it? :P 21 is the legal age, and she's only 18.