If I apply for the Korean citizenship before 35 years old, do I need to serve the military service?
Short answer -- you do not have to, but you can volunteer.
Long answer --
First of all, starting from 2011, the duty to serve is in effect until age 37, not age 35, if you were born after January 1, 1980. Be careful here, however -- the draft eligibility does not expire on your 37th birthday. Instead, it expires on December 31 of the year that you turn 37.
When a male under 37 years of age naturalizes and obtains Korean citizenship, he is eligible for the draft like everyone else. If he does nothing, he will soon receive a draft notice. But a naturalized male Korean citizen can actually file a form and be exempt from military service, except in times of war when he will be drafted for labor mobilization. More about Korea's military service can be found in this series.
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