Final answer:
The achievement of object permanence in infants is marked by their understanding that objects continue to exist even when not directly perceivable. This typically develops between 5 and 8 months of age. Research has demonstrated that infants may understand object properties even earlier than this stage.
Step-by-step explanation:
The achievement of object permanence is best demonstrated by an infant when they can understand that objects continue to exist even when they cannot be seen, heard, or otherwise sensed. This cognitive milestone typically develops between 5 and 8 months of age during the sensorimotor stage, according to Jean Piaget's theory of cognitive development. By around eight months old, children can remember and seek out objects that have been removed from sight, indicating that they have achieved object permanence.
Research has shown that cognitive development in infants includes the ability to recognize objects and their properties even earlier. For instance, infants as young as 3 months old have demonstrated an understanding of the properties of objects without prior experience with them. These findings challenge Piaget's stages of cognitive development, revealing that very young children have a more advanced understanding of objects and how they work than previously believed.