In linguistics, one of the most essential research fields is the existence of the ‘critical period’ in language acquisition. Researchers have tried to find the answers to all the questions arisen in this issue, for instance whether language is instinctive and children can acquire and produce it on their own, without any experience, or language is a more complex process and people need to learn it to be able to speak. Linguists examined many cases to argue for or against these assumptions and to prove their theories; some of them appear in my thesis to support both sides’ assumptions and their different points of view.