Infants younger than six months who have normal hearing can readily distinguish between acoustically similar sounds that are used as part of any language -- not only those used in the language spoken by the people who raise them  Young adults can readily distinguish between such sounds only in languages that they regularly use  It is known that the physiological capacity to hear begins to deteriorate after infancy
So the observed difference in the abilities of infants and young adults to distinguish between acoustically similar speech sounds must be the result of the physiological deterioration of hearing
does not explain the procedures used to measure the abilities of two very different populations