1. A method of assessing a key child's expressive language development, comprising: processing an audio recording taken in the key child's language environment to identify segments of the recording that correspond to the key child's vocalizations; applying an adult automatic speech recognition phone decoder to the segments to identify each occurrence of each of a plurality of phone categories, wherein each of the phone categories corresponds to a pre-defined speech sound; determining a distribution for the phone categories; and using the distribution in an age-based model to assess the key child's expressive language development.