In general, pronouns can refer to any prominent individual in the discourse context. However, a pronoun cannot find its antecedent in certain structural positions, as defined by the Binding Theory. For example, the pronoun "he" can refer to the Ninja Turtle in (1) but not (2), above. Given that speech to children does not indicate what interpretations are impossible, the input is equally consistent with a grammar that allows coreference between "he" and "the Ninja Turtle" in (2) and one that does not. But, since all speakers of English recognize that (2) does not allow this coreference, this aspect of the grammar must come from some property internal to the learner.
