The Stars & Stripes has a good Q&A on South Korea’s mandatory military service:
Q: Who has to serve in South Korea’s military?
A: All able-bodied South Korean men are required to enlist in the military by age 28. K-pop stars aren’t exempt from service, but they have been granted the ability to defer their enrollment until age 30 for contributions to “enhancing Korea’s international image.” At 30 years old, Jin, whose full name is Kim Seok-jin, is the oldest member of BTS and was running out of time to defer, as officials mulled whether to exempt K-pop stars from service. Just weeks before a December enlistment deadline, Jin rescinded his deferral, setting forth his path to military service.
Men with physical limitations or health concerns are allowed to perform alternative service, such as holding administrative roles in the community. Men with religious concerns — most notably Jehovah’s Witnesses — were previously jailed for refusing compulsory military service. In recent years, however, conscientious objectors have been allowed to take community service roles.
Artists and classical musicians who have won awards for their work are allowed to perform alternative service — which requires teaching and performing — as long as the award is officially recognized by the Defense Ministry. (BTS has not won such an award.) Professional athletes who won international honors are also allowed to do alternative service corresponding with their expertise. Korean adoptees who choose to reinstate their nationality — many of whom don’t speak their native language — are granted an exemption from compulsory service.
Q: How long does South Korea’s military service last?
A: Those who are required to serve in the military must do so for at least 18 months, although the length of service can vary by branch. Conscripts serve for 18 months in the army and the marines, 20 months in the navy, and 21 months in the air force. The first few weeks of service are usually spent at a basic training camp before soldiers are moved to the roles in which they will finish the rest of their enlistment.
Stars & Stripes
You can read more at the link.