A common myth in education is that younger students can learn a second language more easily and more quickly than older students. This is a myth that I believed to be true until just a week ago. It has been proposed that there is a "critical period" for children during which the biology of their brains allows them to absorb new language more easily (Lenneberg 1967, in Collier 1987). This hypothesis has been researched at length over the years, and for the most part it has been shown to be incorrect. The reasons given for the difference in language acquisition between age groups are varied. However, there does seem to be agreement that older students have greater cognitive brain development that allows them to have conscious awareness of the language learning process.
Eric Lenneberg in 1967 proposed a critical period hypothesis which explained why children can pick up a new language more easily than adults. He theorized that the acquisition of language is a process determined by biological factors which limit the "critical period" of language learning from about two years old to the age of puberty. Lenneberg believed that once the brain becomes lateralized (when the two sides develop specialized functions), it loses it's plasticity. This means that the brain becomes more "set in it's ways" and it less likely to be adaptable to new ways of thinking. Lenneberg proposed that the lateralization process is completed by puberty, therefore, the ability of the brain to learn a new language after this time becomes decreased. There is some debate however, about when the lateralization process actually occurs. The many studies that attempted to test the critical period hypothesis were focused mainly on one aspect of language proficiency, pronunciation. They were also mainly focused on the differences between adults and children, rather than between age groups of children. As a teacher, I am more interested in learning differences for children and realize that there is a lot more to academic language proficiency than simply pronunciation.