Reasoning on the basis of sentences with the connective unless has only rarely been studied by cognitive scientists. Our investigation starts from the mental model theory of reasoning (e.g., Johnson-Laird, 1983). An assertion such as not-p unless q has the same truth conditions as if p then q, but we argue that it is represented differently, that is, the initial mental models for these two connectives are not the same. The experiment is the first to investigate the role of age (by testing three age groups: nine-year-old, eleven-year-old and thirteen-year-old children) on reasoning with unless. Other investigated factors are type of conditional (if-then vs. unless), and content (by comparing abstract and concrete problems). We discuss the results in terms of recent modifications to the original mental model theory.