Some theories of second language acquisition have had a major influence on the teaching and learning of a second language. This study reports the acquisition of English as a second language over a six-month period by a six-year-old Nicaraguan girl. Findings are based on a cross-sectional study and data that come from observation, journaling, and video recording. There are many beliefs, arguments, and controversies about the importance of being exposed to a language at an early age and the roles that these three vital theories play: behaviorism, innatism and interactionism. Hence, this study sought to investigate the importance of an early start in language acquisition and the role these three core second language theories played in the process. The data collected adds evidence to the theory that age plays a significant role in acquiring a language. The three theories used in this study were found to be present in the process of how this child learned or acquired a language. The use of these theories could serve as a basis for teaching English to children.