Mechanistic complexity is an important property that affects how we interact with and learn from artifacts. Previous research finds that children successfully detect complexity contrasts when given information about the functions of simple and complex objects. However, do children spontaneously favor relevant information about an object’s causal mechanisms and functions when trying to determine an object’s complexity? In Study 1, 7–9-year-olds and adults, but not 5–6-year-olds, favored relevant information (e.g., the difficulty in fixing an object) over irrelevant information (e.g., the difficulty in spelling an object’s name) for making determinations of mechanistic complexity. Only in Study 2, in which the relevance contrasts were extreme, did the youngest age group favor relevant over irrelevant information. These results suggest that the ability to detect which object properties imply complexity emerges in the early school years; young children may be misled by features that are not truly diagnostic of mechanistic complexity.