Object permanence is the cognitive understanding that objects continue to exist even when they are not in sight. According to Piaget, this key cognitive milestone is typically grasped by around 8 months of age. Before developing this concept, infants behave as if an object that is hidden from view has ceased to exist. Once toddlers (12-24 months old) master object permanence, they demonstrate this understanding through behaviors such as enjoying games like hide-and-seek and realizing that a person who leaves a room will return.