  The fact that many adults and older children can learn both first and second languages to a high degree of proficiency makes clear that  unlike the kitten visual system studied by Hubel and Wiesel, the  language acquisition system in humans is not subject to a critical  period in the strict sense. This finding is consistent with the  emerging view that the cortex remains highly plastic throughout life, and that contrary to received wisdom, even old dogs can be quite good at learning new tricks. (See Buonomano and Merzenich 1998; Cowen and Gavazzi 1998; Quartz and Sejnowski 1997; and Stiles 2000.) It is  also consistent with the idea, which seems more plausible  than the critical period hypothesis, that there is a  sensitive period for language acquisition — a time,  from roughly birth age 1 to age 6 or 7, in which language is acquired most easily and naturally, and when a native-like outcome is  virtually guaranteed. (Cf. Mayberry and Eichen 1991.) The  implications of this conclusion for linguistic nativism are examined  in the next section.