APRENDIZADO NA INFÂNCIA – PERGUNTAS E RESPOSTAS
The critical age factor is a much discussed matter. The best and simplest explanation I know is the one given by Yehouda Harpaz (my underlines):
Humans are born with an ability to comprehend and generate all kinds of , but during childhood (starting from birth, and maybe before) such that only the phonemes of the native language are easily comprehended and generated. In adults, these abilities are much less plastic, so adult learners of a new language find it especially difficult to comprehend and generate the phonemes of the new language that are not used in their native language.