In this study, the development of concept generation and mental flexibility was investigated in 84 Australian children between 3 and 7 years of age, using the Object Classification Task for Children (OCTC), a newly developed executive function test for use with young children. On this task, which was adapted from the Concept Generation Test (Levine, Stuss, & Milberg, 1995) and the Concept Generation Test for Children (Jacobs, Anderson, & Harvey, 2001), children were asked to categorize 6 plastic toys according to 3 predetermined groupings (i.e., color, size, and function). The test included 3 performance levels, each providing increasing levels of structure for the child. Findings from the OCTC show meaningful age-related changes in performance across age groups, with older children being less dependent on additional structure to complete the task, in comparison to younger children. Furthermore, findings from this study suggest that the ability to generate concepts emerges between 3 and 4 years of age, continuing to develop beyond the age of 7 years. A developmental spurt in cognitive flexibility was observed around 4 to 5 years of age, with refinement of this capacity occurring between 5 and 7 years of age. Results suggest that the OCTC is a useful measure of conceptual reasoning skills in early childhood.