A brief window of time
Experience changes the brain, and some changes have to occur within a critical period of brain development.
Critical period refers to a brief window of time in the development of the brain when new brain systems and maps develop.
This window occurs early in life, and some important aspects of brain development cannot or will not readily occur after its associated critical period closes. For example, children born with cataracts used to face blindness. However, research revealed that visual stimulation in early infancy is required for newborn brains to get the light necessary to form the brain connections necessary for vision. As a result, infants born with cataracts now receive corrective surgery during this early critical period.
In infancy and early childhood, normal brain development depends on environment, including stimulation from the people interacting with the child or infant. During this critical period, when the brain is also learning how to learn, it is so plastic - i.e., changeable - that learning is effortless, requiring only exposure to new stimuli. The result is rapid formative growth. After the critical period, learning may be unintended, but it is effortful, requiring focus and attention, practice and repetition, and time.
Different brain systems have different critical periods during which the brain is especially changeable and sensitive to its environment. For example, the critical period for language development begins in infancy and ends usually between eight years and puberty. After this critical period closes, learning a new language, and learning to speak it with native fluency, requires more effort.