The child who continue to search for a hidden subject, has achieved the developmental age of

(a). A major milestone is the achievement by 9 mo of object permanence (constancy),the understanding that objects continue to exist, even when not seen. At 4-7 month of age, infants look down for ball that has been dropped but quickly give up if it is not seen. With object constancy, infants persist in searching. They will find obiects hidden under a cloth or behind the examinei's back.