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A sensitive period for acquiring language peaks in the later preschool years, and continues to some degree until 14 to 15 years of age. This special human sensitivity for learning language in the preschool years seems to correspond to certain systematic changes in the brain. The best documented of these changes are called myelogenetic cycles. Each cycle is a period in which myelin forms in a particular system within the brain.There are three myelogenetic cycles in the system that are important to language. The first cycle, which occurs in the brain stem and the limbic system, starts before birth and ends early in infancy. It seems to be associated with the development of babbling. The second cycle, which begins around birth and continues until 4 to 5 years of age, takes place in a more advanced part of the brain. This cycle appears to accompany the development of speech in infancy and the early preschool years. The third cycle takes place in the association areas of the cortex of the brain, which play a central role in intelligence. Although myelination of these areas begins at birth, it is not fully completed until age 14 or later. 
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