Polish-speaking children aged from 2;4, to 4;8 and 16 adult controls participated in a nonce-word inflection experiment testing their ability to use the genitive, dative and accusative inflections productively. Results show that this ability develops early: the majority of two-year-olds were already productive with all inflections apart from dative neuter; and the overall performance of the four-year-olds was very similar to that of adults. All age groups were more productive with inflections that apply to large and/or phonologically diverse classes, although class size and token frequency appeared to be more important for younger children (two- and three-year-olds) and phonological diversity for older children and adults. Regularity, on the other hand, was a very poor predictor of productivity. The results support usage-based models of language acquisition and are problematic for the dual mechanism model.