人类的语言学习有一个由生物因素决定的关键期，它始于二周岁，到青春期结束，过了这个时期，语言学习就会很困难。人的思维机能是有所分工的。左半脑控制语言创造力、句法结构和逻辑思维；右半脑负责非语言创造力的思维活动，如音乐、美术及其欣赏能力、视觉能力和空间思维想像力。一些语言习得研究者所做的实验表明，在自然的语言环境中，如果儿童在五六岁以前开始学习第二语言，那么他们说第二语言则丝毫没有任何口音。对儿童语言发展和外语教育方向的研究是有重要意义的。
儿童语言习得和发展，是指儿童形成对语言的认知，并逐渐提高语言能力的过程。在童年早期，语言能力主要指日常生活中的听力理解与口头会话。儿童在语言习得和发展过程中，通过与家庭成员的交流、对环境的主动探索、及对信息的加工，逐步建构语音、语法、语义和语用方面的基本内容与规律。正常发展的儿童在习得一门语言时，可以不经刻意指导或练习，在五岁前 “毫不费力” 地实现词汇和句法的纯熟运用。与此相对的，青少年和成人学习一门新语言的过程则困难重重。
There is a biologically determined critical period for language learning in humans that begins in the second year of life and ends in adolescence, after which language learning becomes difficult. There is a division of functions in the human mind. The left hemisphere controls language creativity, syntactic structure and logical thinking. The RIGHT HEMISPHERE OF THE BRAIN IS responsible for NON-VERBAL creative thinking activities, such as music, art and their appreciation ability, visual ability and spatial thinking imagination. Some language acquisition researchers have conducted experiments showing that, in a natural language environment, children who begin learning a second language before the age of five or six speak it without an accent at all. It is of great significance to study the direction of children's language development and foreign language education.
Children's language acquisition and development refers to the process in which children form their cognition of language and gradually improve their language ability. In early childhood, language competence mainly refers to listening comprehension and oral conversation in daily life. In the process of language acquisition and development, children gradually construct the basic contents and rules of phonetics, grammar, semantics and pragmatics through communication with family members, active exploration of the environment, and information processing. When a normally developing child acquires a language, he or she can achieve "effortless" mastery of vocabulary and syntax by the age of five without deliberate instruction or practice. On the other hand, teenagers and adults struggle to learn a new language.