From David Lightfoot, The Development of Language, p. 76, note:
“Learning” is a slippery term. Languages are learned in the sense that children derive information from their linguistic environment and then converge on some grammar in accordance with specific principles. That, however, is a specialized sense of “learning”, and not the way the term is used in informal discourse. If we understand learning in a nontechnical sense, then it is not clear that learning plays any role in the acquisition of language. In that case, we may be better off not to talk about children “learning” a language, but to talk instead in terms of the growth of language. “
If we consider the possibility that second language learning as adults (12 on up) activates the same neural mechanisms that first language learning does, we see the basis for CI. I’ve been reading some of the posts about teaching grammar on flteach and Cambridge Latin and those teachers are writhing in agony trying to get their students to put the right endings on adjectives and parse relative clauses and use the reflexive for something other than brushing their hair.
This Lightfoot book is no easy read and this is not a book in applied linguistics where the author has any interest in teaching languages.