The Online Dictionary of Language Terminology (ODLT)

Critical Period Hypothesis
Definition - (Aka CPH | Critical Age Hypothesis)

The hypothesis that (1) the first few years of life are the crucial time for individuals to acquire a first language and that (2) after a certain age (approximately 5-years) language learning is more and more difficult.

1. Evidence for the hypothesis:
(1) The existence of feral children who couldn't learn to speak after being deprived of early linguistic input.
(2) Adults are more likely to suffer permanent language impairment from brain damage than are children because the latter's brains can adapt to the changes.

Etymology -
The hypothesis was first proposed by Wilder Penfield and Lamar Roberts in their 1959 paper Speech and Brain Mechanisms. It was made popular by Eric Lenneberg in his 1967 book Biological Foundations of Language.