Krashen, S. (1982). Second Language Acquisition Theory. In Principles and Practice in Second Language Acquisition. Retrieved from http://www.sdkrashen.com/content/books/principles_and_practice.pdf
In this chapter, Krashen revises aspects of the relationship between age and second language acquisition and concludes that despite the fact that children are superior to adults in the long run, older acquirers are superior when it comes to rate of attainment. His assumptions rely on the postulates of the Input Hypothesis proposed in the 1970s which claims that second language acquisition takes place when comprehensible, interesting/relevant and not grammatically input is provided.
Studies mentioned by Krashen pointed out that older acquirers like adolescents could better regulate the quantity and quality of their input when communicating with native speakers; other studies showed that the child’s superiority in attainment is related to the strengthening of the affective filter at puberty.
In summary, Krashen’s hypothesis states that the process of L2 acquisition occurs almost in the same way as L1 acquisition regardless of learner’s age. Although he acknowledges that input is harder to be understood by adults since their world is more complex than the children’s, he argues that age itself is not the only predictor of a successful second language acquisition process but also the existence of a comprehensible input and the level of the affective filter.
When it comes to relating the Input Hypothesis and foreign language teaching materials, one must consider that age plays an important role in the development of resources that would be able to provide input and help students in language acquisition. First of all, it is worth pointing out that the theory focuses on the development of materials for beginning and intermediate students and, it also states that listening and reading materials should address interesting and relevant topics. Therefore, it is important to take into account when producing language materials that students from different ages consume different types of texts and are interested in different daily-life topics. The role of materials producers is to try to meet the requirements of the Input Hypothesis so students can be provided with materials that would encourage acquisition and help them obtain comprehensible input outside the classroom.