Final answer:
Object permanence typically develops in infants around six to eight months of age. It marks a key cognitive milestone where a child understands that objects continue to exist even when not visible.
Step-by-step explanation:
Object permanence is the understanding that objects continue to exist even when they are out of sight, and this cognitive milestone develops in infants typically around six to eight months of age. During the sensorimotor stage of development, described by psychologist Jean Piaget, young children explore the world through their senses and motor actions. It's agreed upon that as they mature, infants begin to grasp the concept of object permanence, where they realize that objects, and by extension people, have a continuous existence even when not directly observable. This developmental achievement allows them to anticipate the return of a person who leaves the room or to search for a toy that has been hidden from their view.