A common approach in the theoretical literature on language is to characterize its organization: the mechanisms deployed in understanding it, the representations over which these mechanisms operate and, sometimes, the computational architecture in which these processes occur. For the most part, work of this kind has focused on capturing what is representative of a typical mature language user, and a typical developmental path to that end state.
In general, differences among language users at each stage of development are ignored. Data used to inform theory building and testing, whether observational or experimental, are most often characterized by an emphasis on normative behavior. Variability around the typical case is usually treated as observational noise, a nuisance.
I would like to explore the idea of taking explanation of variability in language organization as a primary goal of theory. Plasticity and learning must have a central role in such a theory. Only a model that takes plasticity seriously can provide a mechanistic account of both process and representation in language, and the means by which these change over the course of development: a necessary property of any theory meant to explain variation in the cognitive organization of language.