We argue the case for general cognitive-processing constraints on, and cognitive-developmental stages in, linguistic performance. We use the theory of constructive operators of Pascual-Leone to analyze the sorts of’software’ (i.e., content-bound) and ‘hardware’ (i.e., content-free organismic) processes relevant for a cognitive developmental model of language. Emphasis is placed on the hardware: mental-attentional capacity as a limit on linguistic competence. We describe types of situations in which the effects of such capacity should be clearly manifest in performance and support our claims with results of two studies on language development, one on comprehension and production of subordinate conjunctions and the other on metaphor interpretation. Various linguistic performance scores in the two studies increased with age in steps that correspond to theory-predicted stages in the growth of mental-attentional capacity; a similar stage-bound developmental pattern is reported for a visual information-processing task. Theoretical and methodological implications are discussed.