According to the critical period hypothesis in linguistics (CPH), the ability to learn languages is confined to a biologically circumscribed period, ranging from the age of about 18 months up to the age of adolescence. The notion of a critical period finds its inspiration in the biological concept of maturation, whereby development is programmed to follow a genetically-determined sequence. However, the expression of the genetic programme requires appropriate environmental conditions. Hence, it is argued that in the absence of normal exposure to language during the critical period, it would subsequently be biologically impossible to acquire language normally.

The CPH predicts that brain damage which would cause language loss in adults should not have such catastrophic effects in infancy. This is because the child\'s brain is still being moulded by maturational forces and the concomitant flexibility allows other regions of the brain to take over and proceed with development more or less normally. However, it has emerged that specific brain injuries in infancy can result in permanent language impairment. Furthermore, the flexibility of younger brains does not entirely disintegrate in adulthood. Currently, in fact, there is no convincing evidence for a decisive cut-off point at adolescence, beyond which language learning is impossible.