Mutualism is a developmental theory that posits positive reciprocal relationships between distinct cognitive abilities during development. It predicts that abilities such as language and reasoning will influence each other's rates of growth. This may explain why children with Language Disorders also tend to have lower than average non-verbal cognitive abilities, as poor language would limit the rate of growth of other cognitive skills. The current study tests whether language and non-verbal reasoning show mutualistic coupling in children with and without language disorder using three waves of data from a longitudinal cohort study that over-sampled children with poor language at school entry (N = 501, 7-13 years). Bivariate Latent Change Score models were used to determine whether early receptive vocabulary predicted change in non-verbal reasoning and vice-versa. Models that included mutualistic coupling parameters between vocabulary and non-verbal reasoning showed superior fit to models without these parameters, replicating previous findings. Specifically, children with higher initial language abilities showed greater growth in non-verbal ability and vice versa. Multi-group models suggested that coupling between language and non-verbal reasoning was equally strong in children with language disorder and those without. This indicates that language has downstream effects on other cognitive abilities, challenging the existence of selective language impairments. Future intervention studies should test whether improving language skills in children with language disorder has positive impacts on other cognitive abilities (and vice versa), and low non-verbal IQ should not be a barrier to accessing such intervention.