Actually, here's an extract from a well informed piece of research...
Son could do his military service aged 33....
"the current rule for all able-bodied Korean men is that they must serve a military duty that lasts for about 2 years before the age of 35.
Each branch of the military has a different length of service: 21 months for the Army and Marines, 23 months for the Navy, and 24 months for the Air Force. For most Korean men, they elect to serve their duty right after graduating high school and put off university entrance for 2 years so they can study in university and begin their career unburdened. However, for footballers, the habit is a little bit different and presents a lot of complex dilemmas.
For footballers, the Korean Armed Forces Athletic Corps has Sangju Sangmu FC and the Korean Police Agency has Asan Mugunghwa FC. These two clubs accept domestic-based players on a rolling basis and conscript players for a 21-month period loan. Sangju is currently playing in the K League 1 and Asan is in K League 2, although Asan’s strong midfield class that includes KNT call-ups Ju Se-jong and Lee Myung-joo has them in the Promotion Play-off places so we could see them both in the top division very soon.
How do you get selected for these teams? There are a couple rules and we will be examining the shortcomings of the current rules in place. The first rule is that both clubs are only allowed to pick players currently playing in the K League. Even more than that, the player has to have been active in the K League for the past six months. This was why Lee Myungjoo finished out last season with FC Seoul before starting his service with Asan. Another rule is that you must begin your service before the age of 27 (28 Korean Age) to serve with one of these clubs. If you wait until after that deadline, you will serve in a normal military unit and will not be able to train with these football clubs during your service. As it stands, most Korean footballers serve at Asan or Sangju in their mid-twenties after playing a few seasons in the K League. For players who play abroad like Lee Myungjoo, they return to the K League around that time and sign for a K League club before getting loaned. For the most part, it is rare for a player to return to a career abroad after finishing their military service.
Exemptions: How do you get one?
Thankfully, the Korean Military Manpower Administration has laid out some athletic accomplishments that allow a footballer to become exempt from conscription. The 2002 World Cup heroes who achieved 4th place at the World Cup hosted in South Korea and Japan were given exemptions.
However, this rule doesn’t seem to be permanent as there hasn’t been another World Cup squad given exemptions. It might be up to the current administration to decide if the World Cup squad has done well enough to deserve exemption (paging Moon Jae-in).
The rule that is set into law is that an Olympic medal or an Asian Games gold medal is enough to allow any male athlete exemption from military service.
The 2012 bronze-winning side and 2014 Asian Games gold medalists are all military exempt. Ki Sung-yeung himself has said that this was key to allowing this generation of Korean footballers the chance to pursue careers in Europe.
Ki once went home during the offseason to do a month-long training spell with the army but that’s about it. Think about that! To go from 21 months to just one month is a huge gift!
There are also some loopholes in these military conscription rules that Korean footballers have tried to exploit, to some success and controversy. For example, Lee Chung-yong was spotted by FC Seoul as a middle schooler and subsequently dropped out of school to join their youth sides. This was because at the time the Korean military did not draft men who hadn’t completed high school so as a result Lee has never had to do military duty. However, that loophole has since been closed. A controversy arose when Park Chu-young tried to argue that his residency visa in Monaco allowed him to postpone his military duty until he was in his 30s. He became quite unpopular in Korea when he tried this and was only able to avoid military service by being chosen as an overage wildcard for the London Olympics and then scoring a goal in the bronze medal match. These are the most prominent examples of players trying to skirt the rules but I am sure there are more examples. "