Final answer:
Leonardo has a lower threshold for detecting dirty diaper odors than his grandparents, meaning he can perceive the smell at lower concentrations.
Step-by-step explanation:
Leonardo must have a lower threshold for dirty diaper odors than his grandparents have. A threshold in this context refers to the lowest concentration of a substance that can be perceived by the sensory receptors. Having a lower threshold suggests that Leonardo is able to detect the odor at lower concentrations, which might be due to a number of factors including age, health, and individual sensitivity.