Is it really true that if we don’t hear a language by the age of eight, we’ll probably never learn to speak?
Yes. We are born with a predisposition for language – but it has to be activated by the world around us. We now know that every baby has the neurological structure which enables it to make about 60 structured sounds (phonemes). Combined, these phonemes create all the sounds used in the world’s languages. A child growing up in a Western type environment is only likely to use half the available phonemes; those phonemes it doesn’t use gradually waste away. Fortunate is the child who grows up in a multi-language culture (as in central Africa) who can hear, and then copy, upwards of half a dozen languages at any one time… so retaining and practicing more phonemes than a child within a single language.
See Cavalli-Sforza L. (2000) Genes, Peoples and Languages, Allen Lane and Pinker S. (1994) The Language Instinct: The new science of language and mind, Allen Lane"