Summary Aim of the study to discuss and clarify that if there is a critical or sensitive period in the process of the acquisition of first or second languages Research Questions Does language acquisition progress normally and does the child display a full proficiency in the language when he she begins to learn at an early age Context Participants and Methodology The context of this study is first and second language learning ESL English as a Second Language Participants are just existing through examples there is no study on a specific group of people They are random people who are learning a second language for example immigrants who moved to the United States from China and Korea Findings of the study Suggestion from Eric Lenneberg 1976 human language acquisition is an example of biologically contrived learning If a person acquire the language in his her critical period beginning with childhood and ending with puberty his her language proficiency can be at the maximum degree but if language is acquired after critical period there will be a different or more difficult learning process For him critical period ends because the brain reaches its mature organization in the last stage of adolescence