Based on his analysis of existing clinical literature on unilateral brain damage and hemispherectomies, he concluded that there is progressive lateralization of the language function to the left hemisphere, a process which is complete by puberty. He linked this putative phenomenon to what he believed to be a dramatic decrease in the ability to acquire language.
The Critical Period Hypothesis lends support to why it is important to begin L2 instruction K-8! I am interested in learning more about the "Windows of Opportunity." It may become more difficult for adult language learners to have native-level fluency in L2 if they start after the Critical Period, but what level of proficiency CAN they achieve? The goal is not always native-level fluency.