You must be at least 9 months old to walk up the stairs
Where did the plane go?
The 5 parts of Cognitive Development we have learned so far are: Maturation, Object permanence, Critical periods, Assimilation, and Accomodation
Maturation is biological patterns of behavior present in a species. In this scenario, the child is unable to walk, let alone climb the stairs until they are a certain age.
Object permanence is the understanding that objects continue to exist even when they cannot be seen. The older child understands that the plane is gone, and he is upset that he cannot find it, while the younger child has forgotten it even existed.