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.
Note: I had New Zealand researched as 18 with adult, 21 alone - but several New Zealanders have written me to say you really can drink in public if you're 18.