Age is an essential factor in Second Language Acquisition (SLA), impacting the success of students and instructional methods. The purpose of this study is to examine the age factor in SLA by examining three age categories -- children, adolescents and adults. In doing so, the study considers the Critical Period Hypothesis as a base of linguistic research in the area of age factor. The study disapproves the assertion of the hypothesis that all prepubescent learners are able to acquire native-like proficiency in target language pronunciation. The study analyzes common SLA beliefs, including: 1) younger learners are more successful than older learners, 2) the language learning processes of younger learners are less stressful and require less of an effort, and 3) young learners are more skillful in language learning. Adolescents and adults are considered as older learners in terms of cognitive maturity. The results of the study indicated that children learn a language easier than adolescents and adults, particularly with respect to pronunciation and morpho syntax. Adolescents are good at syntax and listening sills, while the best results for adults are for reading and writing activities. Thus, the types of brain organization at learners of different developmental stages lead to the need for a diversity of instructional methods for children, adolescents and adults.