Final answer:
Wally, showing awareness of differing beliefs that could be false, is likely about four to five years old, reflecting his development of theory of mind (TOM). This cognitive milestone occurs between the ages of 3 and 5 years old.
Step-by-step explanation:
Wally has just begun to understand that people may hold some beliefs that are not true. If Wally is like most other children, he is about four to five years old. This understanding is part of a crucial cognitive development stage known as theory of mind (TOM), which is typically developed between ages 3 and 5. Theory of mind allows children to recognize that others have different thoughts, feelings, and beliefs, and that these beliefs can be false. False-belief tasks are exercises that can help determine a child's acquisition of theory of mind.