This study examined age differences in young people's understanding of evolution theory in secondary school. A second aim of this study was to propose a new coding scheme that more accurately described students’ conceptual understanding about evolutionary theory. We argue that coding schemes adopted in previous research may have overestimated students’ grasp of evolutionary concepts. A total of 106 students aged 12, 14, and 16 took part in individual interviews investigating their understanding of evolution. Using the new coding scheme, we found that while 16‐year olds were more likely than 12‐year olds to endorse scientific concepts when answering a question about finches, their understanding of natural selection, however, did not generalize to the other four questions. Furthermore, students began to incorporate relevant terminology (e.g., adapt, evolve, etc.) and structure their explanations using relevant language at around age 14. Students often used relevant terminology without having a more advanced understanding of evolutionary theory. Instead, they used the relevant terms in a colloquial rather than a scientific sense. Implications of the current findings for teaching and theory are discussed. © 2016 The Authors. Journal of Research in Science Teaching published by Wiley Periodicals, Inc. on behalf of National Association for Research in Science Teaching. J Res Sci Teach 54: 247–273, 2017