Denmark
In Denmark, the twelfth grade is the 3rd G, its equivalent is the 3rd grade of secondary school, or gymnasium. This is not compulsory, as education is only compulsory through 9th grade. Students are often 18-19 because there is a one year gap (it is optional; you can go directly to the first year of secondary school) between the 9th grade and gymnasium's 1st G were students typically go on a trip or go to some special easy going art and sport oriented schools called "afterschool". Sometimes it is also because Danish children first start school at 6 or 7.
Read more about this topic: Twelfth Grade