Normal primary language acquisition starts with acoustic input in the prenatal phase. Within a few years, the child attains a competence sufficient for his own standards; i.e., he can use his vocabulary and execute linguistic operations to say everything he is cognitively capable of. The language learner reaches a steady state about the beginning of puberty. At this point, adolescents essentially possess full competence in their primary language. This does not, of course, exclude the possibility of continuing to learn during one's entire lifetime. This applies primarily to new vocabulary, less so to rules and formatives of grammar.

During the maturation phase, the brain grows in size and internal structure. At this time, the human being has a greater learning capacity than as an adult. The period in which a human being easily acquires native competence in languages ends at the beginning of puberty. Languages learned afterwards will generally be spoken with an accent. The period from birth to an age of about 12 years is therefore called the critical or sensitive period.

This point is often overstated. The critical period is not an absolute delimitation of an innate biological capacity of acquiring competence in a language. The following restrictions have to be made:

• Individuals differ considerably in their ability to learn languages. Some acquire with ease full competence in additional languages even at an adult age.
• The critical period is the same period in which primary language acquisition occurs. In all societies in which relevant research has been done, the learning process is actively and constantly supported by caretakers who provide instant, dedicated and reliable feedback. Such conditions generally no longer obtain at a later age. After the critical age, languages are usually learnt under much less favorable conditions.
• In most societies were relevant research has been done, the initial and largest portion of the critical period coincides with the period of monolingual primary language acquisition. During it, categories are formed and processes are automatized that define boundaries and deeply entrenched habits. They get wired in the neural system and are very hard to release again so that the mind becomes open for new categorizations and operations. This has little to do with a particular age span; it is true for every learning process that is first confined to only one specimen of a kind.

The critical phase has been said to restrict not only the ability to learn a language, but more fundamentally the ability to acquire human language. This claim is based on children who did not, for individual or social reasons, have a chance to learn the language of their environment. These include “wild” children, (criminally) neglected children or children born deaf. In several documented cases, such children came to enjoy a more propitious environment at an adolescent age, but were then unable to learn speaking.

Such cases may be evidence for a time window for the acquisition of human language which closes at adolescent age. However, they are fraught with considerable methodological problems. If a person gets no chance to communicate until youth, lack of language is not his only problem. If then they are unable to learn a language, the reasons for it may be manifold and not limited to having passed the sensitive period.