Comment by Capitulate on Mercenary Enrollment - Chapter 85

 In Korea, you are 1 year old at birth. Most, if not all, the rest of the world starts at 0 years old when born. 
That crossed out sign was the only thing incorrect, it should have read 20. Otherwise, everything else in the story is in Korean years. 

That said, this is just a theory. Though, it's the only way to make sense why 19 year olds can't drink when the drinking age is 19 (in international years).

To further support the theory, it's pretty common that Manhwa use Korean years. There are certainly exceptions to this, and it can definitely be confusing. If you look up the drinking age in Korea, it is 20 (Korean years), but the Manhwa's crossed out sign said 19. With no other evidence it would suggest that the whole story was in international years. However, 3rd years in Korean HS are 17-18 (International aka 18-19 in Korean years). They are still in school and have suggested they are below the drinking age, and thus the sign must be the error. 

I hope this can clear up any confusions.