Language acquisition is an interesting mix of
linguistics,
psychology, and
biology; over the centuries that scientists have been
trying to answer the question, "how does a child learn a language," one theory of note has emerged: the Critical-Age
Hypothesis.
It is well known that children pick up the language of their home environment by imitation and unconsciously learning the grammatical
rules and structure of that language. Innumerable studies show that children are, without actually being able to explain how,
capable of, for example, pluralizing words they have never heard. Ask a child, "what do you call more than one doob?" He or she will
invariably answer, "doobs." This is language acquisition by young minds at work.
There is, however, a limit, a statute of limitations, so to speak, on early language acquisition: This is the heart of the critical-age
hypothesis. Children are only able to learn the structure of a language, and, more basically, the fundamentals of human communication
and universal syntax, from birth to puberty, the latter being generalized to about 12 years old. What does this mean? If a child cannot
speak a language coherently (speech impediments and just plain old stupidity aside) by the time he or she hits puberty, he or she will
never be able to speak a language.
This is a tried and true theory; there is something that must develop in the Broca and Wernicke areas of the brain early on in life
that can only be accomplished by exposure to and assimilation of a language. There are famous cases of "wild children," like the well known
Victor in Truffaut's film, "The Wild Child," that were raised either in the wild (as in Victor's case) or just cut off from humanity
(as in the terrible case of Genie). All of these subjects were unable to grasp language; Victor, taken in by a physician and linguist,
never progressed passed very simple French (think first year high school French) and grunts and pointing. Genie, with more modern
methods, was able to speak, but often unintelligibly. In fact, language acquired after the critical age is almost exactly like the
language spoken by patients stricken with Broca's aphasia: strings of nouns with some basic verb use, but no concept of tense,
auxiliary verbs, or anything that would make the language ultimately understandable.
It is very important to note, however, that the critical-age hypothesis does not apply to all of a language. In fact, hundreds
of thousands of people a year (a figure I just made up, but it seems reasonable) acquire fluency in a second language without the
aphasia-like symptoms described above. Why? This hypothesis applies only to certain elements of language, not a language.
Specifically, it applies to certain elements of grammar that are common to all human languages, and without this development of the
Wernicke's area, speech is incomprehensible. Simply put, beyond a certain age, the human brain is not able to acquire inflectional
morphology or much of syntax.