The optimal age in FLL (foreign language learning) for children has been discussed over 50 years but there is no satisfactory conclusion for us. However, the notion "the younger, the better" in FLL has a big market in the world. As a result, the distorted hypothesis is being spread widely as a true and complete theory. Specifically speaking, it's caused by the confusion on the concepts of "second language and foreign language", "learning and acquisition", "CPH (critical period hypothesis)". Therefore, based upon the discussion of theoretical foundation of linguistics, psychology and physiology in FLL for children, the environment of FLL and the importance of mother tongue, all of us will have a complete knowledge of the concept. By analyzing the deep reasons on the tendency of lowering the age in FLL both at home and abroad, those blind followers who are still being misled will have a rational attitude towards FLL. Hence, the rational deeds of "a language can be taught from any age upwards" will definitely go into the heart of everyone.