<P> Studies related to vocabulary development show that children's language competence depends upon their ability to hear sounds during infancy . Infants' perception of speech is distinct . Between six and ten months of age, infants can discriminate sounds used in the languages of the world . By 10 to 12 months, infants can no longer discriminate between speech sounds that are not used in the language (s) to which they are exposed . Among six - month - old infants, seen articulations (i.e. the mouth movements they observe others make while talking) actually enhance their ability to discriminate sounds, and may also contribute to infants' ability to learn phonemic boundaries . Infants' phonological register is completed between the ages of 18 months and 7 years . </P> <P> Children's phonological development normally proceeds as follows: </P> <P> 6--8 weeks: Cooing appears </P> <P> 16 weeks: Laughter and vocal play appear </P>

When does vocabulary develop slowly and when does it develop quickly