Do not believe THE CLAIM that foreign adults cannot master the Korean language. The video link below is evidence that they can.
The video is an interview with 42-year-old, American John Frankl, who is an Associate Professor of Asian Literature at the Underwood International College (UIC) at Yonsei University. He started studying Korean in 1987 while majoring in English Literature at UC Berkley. He came to Korea as an exchange student in 1988 and studied for one year at Yonsei University. When he finished his undergraduate program at Berkley, he came back to Yonsei and started a Master's course in Korean Literature. He got his Master's in 1993 and then went to Harvard University, where he got his Ph.D. in 2003. He became a professor at UIC in 2005. He is not only fluent in Korean, he also has a black belt in Jiu Jitsu. Information on John Frankl
Video of Korean Interview with John Frankl