Critical period

There is a specific period during which children can acquire language easily without the aid of any formal language instruction. Eric Lenneberg (1967) first proposed that the ability to learn a language develops within a fixed period, from birth to puberty. The notion of critical age is true in many species and seems to pertain to species-specific, biologically triggered behavior. Ducklings, for example, during the period from nine to twenty-one hours after hatching, will follow the first moving object they see, whether or not it looks like a duck. Such behavior is not affected by conscious decision, external force or intense practice (Fromkin & Rodman, p. 342). Goodluck maintains that human's ability to learn a language is "significantly impaired" beyond the critical period (p. 141).

http://www.let.uu.nl/~wim.zonneveld/personal/phonologyinnateug.htm

[Sam: The critical period for the development of voice recognition system appears to be from birth to the 9th month. Beyond this period, the infant is no longer able to input foreign sounds into one's voice recognition system automatically. However, the child can still do it with the help of a native teacher. The efficiency drops with age.]