The critical period hypothesis is a hypothesis within the field of linguistics and second language acquisition that claims a person can achieve native-like fluency in a language only before a certain age. It is the subject of a long-standing debate in linguistics and language acquisition over the extent to which the ability to acquire language is biologically linked to developmental stages of the brain. The critical period hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in their 1959 book Speech and Brain Mechanisms, and was popularized by Eric Lenneberg in 1967 with Biological Foundations of Language.
The critical period hypothesis states that the first few years of life is the crucial time in which an individual can acquire a first language if presented with adequate stimuli, and that first-language acquisition relies on neuroplasticity of the brain. If language input does not occur until after this time, the individual will never achieve a full command of language. There is much debate over the timing of the critical period with respect to second-language acquisition (SLA), with estimates ranging between 2 and 13 years of age.