Criticism of the Critical Period Hypothesis

It states that after this period, certain aspects of language may be learned but never fully mastered and acquisition is increasingly difficult.
Key figures include Wilder Penfield and Eric Lenneberg
The critical period ends approxamitely when a child reaches puberty and the cerebral hemispheres separate and their functions become set
Associated concepts include operant conditioning and universal grammar
Born with a severe hearing impairment, she was raised without being able to listen to others speak and interact with them.

If so, lateralization cannot be the evidence of the critical period.

One is that children's acquisition of a foreign language is different from that of adults'.

Critical period hypothesis - Wikipedia

For all the poverty of the stimulus argument shows, the constraints in question might indeed be language-specific and innate, but with contents quite different from those proposed in current theories of UG. Or, the constraints might be innate, but not language-specific. For instance, as Tomasello 2003 argues, children's early linguistic theorizing appears to be constrained by their inborn abilities to share attention with others and to discern others' communicative intentions. On his view, a child's early linguistic hypotheses are based on the assumption that the person talking to him is attempting to convey information about the thing(s) that they are both currentlyattending to. (Another example of an innate but non-language specificconstraint on language learning derives from the structure of the mammalian auditory system; ‘categorical perception,’ and is relation to the acquisition of phonological knowledge is discussedbelow, §3.3.4.). Another alternative is that the constraints might be learned, that is, derived from past experiences. An example again comes from Tomasello (2003). He argues that entrenchment, or the frequency with which a linguistic element has been used with a certain communicative function, is an important constraint on the development of children's later syntacticknowledge. For instance, it has been shown experimentally that the more often a child hears an element used for a particular communicative purpose, the less likely she is to extend that element to new contexts. (See Tomasello 2003:179).

Critical Period Hypothesis | Language Acquisition | …

Suppose that the primary linguistic data were impoverished in all theways that nativists claim and suppose, too, that children know a bunch of things for which there is no evidence available — suppose, as Hornstein and Lightfoot (1981:9) put it, that “[p]eople attain knowledge of the structure of their language for which no evidence is available in the data to which theyare exposed as children.” What follows from this is that there must be constraints on the learning mechanism: children do not enumerate all possible grammatical hypotheses and test them against the data. Some possible hypotheses must be ruled out a priori. But, critics allege, what does not follow from this is any particular view about the nature of the requisite constraints. (Cowie 1999: ch.8.) A fortiori, what does not follow from this is the view that Universal Grammar (construed as a theory about the structural properties common to all natural languages, per Terminological Note 2 above) is inborn.

He claimed that children before their critical period were less severely impaired by brain damage.
Can they outperform the learners who started learning a second language before puberty?

Critical Period for Language Development - SLT info

This toy example illustrates a further route by which language might have evolved in human beings. In addition to creating inborn language-learning mechanisms in individuals, natural selection may also have created dispositions to construct particular kinds of linguistic learning environments in their parents. For example, as Clark (1996) and Sterelny (2003) both speculate, Mother Nature might have worked on our dispositions to use ‘Motherese’ to our children, and/or on our tendency to talk about things that are current objects of the child's perceptual attention, in order to create learning environments conducive to the acquisition of language.

Phonological acquisition seems to be more sensitive to the critical period than that of grammar.

The critical period hypothesis is the ..

In this section, we will focus on features (v) and (vi) of the pld. For it is consideration of the positivity of the data set, and the lack of feedback available to children, that has given rise to what I am calling the ‘Unlearning Problem,’ otherwise known (somewhat misleadingly) as the ‘Logical Problemof Language Acquisition.’ (For statements of the argument, see,e.g., Baker 1979; Lasnik; 1989:89-90; Pinker 1989.)

'A critical Period for Learning to Pronounce Foreign Language?' Applied Linguistics.

Critical Period Hypothesis & Language Acquisition by …

Critical Period: a time during development which is literally critical; the relevant competence either cannot develop or will be permanently lost unless certain inputs are received during that period.