I think the inability to pronounce certain sounds, English “r” for example among speakers of Japanese, Chinese, comes about with age. As I think we all know, young children brought up in bilingual environment can usually pronounce, better yet, speak, both languages correctly. There is some suggestion – how valid I do not know – that baby talk, that babble of seemingly meaningless syllables, consists of an infant’s trying out a hardwired succession of sounds to figure out which ones it will need; as the child grows older and speaks the language, he no longer has a need to pronounce sounds not present in that language and therefore cannot readily reproduce them. (I think Pinker has made this argument, and it makes sense to me, but, once again, I am not a linguist.) This may well explain why learning a language later in life almost invariably produces speech in the new language that is deficient in several respects, primarily pronunciation, but also incorrect sentence rhythm, pacing, etc.

Lee