Subject pronoun use in adult Spanish is strongly influenced by Continuity of Reference, which refers to whether a verb maintains the same subject as the previous verb (same-reference) or changes it (switch-reference). Overt subject pronouns are used more frequently in switch-reference than in same-reference contexts. To investigate the development of this discourse predictor of pronoun use, a preference task was given to 149 children, ages 5;9 to 15;8, and 30 adults in Queretaro, Mexico. Participants were told stories that contained either a same- or switch-reference context. In these stories, overt pronouns in same-reference contexts were redundant, and null pronouns in switch-reference contexts were ambiguous. Adults strongly preferred overt pronouns in switch-reference contexts and null pronouns in same-reference contexts. Many of the youngest children, however, preferred null pronouns in switch-reference contexts, revealing their difficulty with establishing clear referents for pronouns. By age nine, children significantly preferred overt pronouns in switch-reference contexts, but over-accepted overt pronouns in same-reference contexts. By age 14, children preferred null pronouns in same-reference contexts, but not to the degree adults did. The results are interpreted within the context of the "interface hypothesis" (Sorace 2005), according to which features at the syntax-discourse interface are more complex and develop later than features regulated by syntax alone.