In Korea, the school year starts in March and ends in February, and thus people born in the early part of the year (January-February) are put in a grade level above. For example, Kyungsoo is technically "a year" younger than Chanyeol, but he's still in the same grade as him since he's born in January. The reason for the hyphen for the ages is that since the story will take place throughout the course of the school year, their ages are subject to change. 

Kim Minseok: 19-20; high school drop-out; convenience store worker.
Lu Han: 19-20; first year of college.
Wu Yifan: 18-19; third year of high school.
Kim Junmyeon: 18-19; third year of high school.
Zhang Yixing: 18-19; third year of high school.
Byun Baekhyun: 17-18; second year of high school.
Kim Jongdae: 17-18; second year of high school.
Park Chanyeol: 17-18; second year of high school.
Do Kyungsoo: 17; second year of high school.
Huang Zitao: 16-17; first year of high school.
Kim Jongin: 16; first year of high school.
Oh Sehun: 15-16; final year of intermediate school.