In South Korea, all men over the age of 18 are required to serve in the military for a minimum of 18 months by the age of 28. But in 2017, Ah-In was determined to be unfit for service (despite him wanting to serve his country as an active-duty soldier). In addition to having a benign bone tumor, Ah-In had sustained injuries after rupturing his shoulder muscle twice while filming the action flicks Tough as Iron and Veteran.