Recent neurobiological studies have advanced the hypothesis that language development is not continuously plastic but is governed by biological constraints that may be modified by experience within a particular time window. This hypothesis is tested based on spontaneous speech data from deaf cochlear-implanted (CI) children with access to linguistic stimuli at different developmental times. Language samples of nine children who received a CI between 5 and 19 months are analysed for linguistic measures representing different stages of language development. These include canonical babbling ratios, vocabulary diversity, and functional elements such as determiners. The results show that language development is positively related to the age at which children get first access to linguistic input and that later access to language is associated with a slower-than-normal language-learning rate. As such, the positive effect of early experience on the functional organisation of the brain in language processes is confirmed by behavioural performance. © 2011 Psychology Press.