Purpose: Critical thinking pervades formal educational benchmarks in the United States, including the Common Core State Standards. However, little information is available on how it develops. Hence, the primary purpose of this study was to examine the development of critical thinking in adolescents using a written language-sampling task. We also examined related aspects of development: verbal productivity, syntactic complexity, and metacognitive verb use. Method: The participants included two groups of adolescents, aged 13 and 16 years (n = 40 per group). All testing took place in classrooms at a middle school or high school. Participants read four fables by the Greek storyteller Aesop (circa 620-560 B.C.) and explained in writing why they agreed or disagreed with the moral message of each story. To examine critical thinking, we evaluated their explanations using a unique 0- to 4-point scoring system. We also examined each participant's transcript for verbal productivity, syntactic complexity, and metacognitive verb use. Results: On the critical thinking task, the 16-year-olds outperformed the 13-year-olds, providing explanations that were more elaborate and detailed. However, there were many individual differences within groups, and even the older group did not consistently perform at the highest level, indicating that critical thinking is a late-developing ability. Age-related gains also occurred on verbal productivity and metacognitive verb use but not on syntactic complexity. Conclusion: Information gleaned from this study demonstrates how critical thinking develops during adolescence but remains incomplete. The study also has implications for assessing critical thinking in adolescents and knowing how to prompt complex language and thought.