In developmental psychology, we often try to find qualitatively distinct differences as a person develops over time. We have a habit of looking for stages that occur but it’s not as if one day a child is crawling, then the next day they are sprinting across the floor. Development is more gradual than that, of course. Sometimes these changes can feel rather sudden, especially when you are a relative who visits a young child. Each time you see them they will be doing something that they could not possibly have been able to do the last time you saw them.
Premack, D. & Woodruf, G. (1978). Does the chimpanzee have a theory of mind? Behavioral and Brain Sciences, 1, 515-526.