I really believed this. As I grew up, I knew this was wrong but nevertheless persuasive. Many Korean-Americans speak English very well, as well as Americans. The reason is due to practice from infancy. Most people learning other languages besides their mother tongue experience great difficulty, especially in the cases of adults.
People say that children have an unconventional tongue so they can learn languages easily.
On the other hand, adults who spend a lot of time in their native country can't pronounce other languages well. However, sometimes we can find people fluent in English even though they are adults. I think it depends on an effort to improve pronunciation. Surely it's so difficult that most adults can't do that without worrying about their pronunciation. I am one of these people.