Developmental Variability and Readiness
The tendency to exhibit individual differences is crucial. Each person is unique with his or her own timetable for development. This timetable is a combination of a particular individual's heredity and environmental influences. Although the sequence of appearance of developmental characteristics is predictable, the rate of appearance may be quite variable. Therefore, strict adherence to a chronological classification of development by age is without support or justification.
The "average" ages for the acquisition of all sorts of developmental tasks, ranging from learning how to walk to gaining bowel and bladder control, have been discussed in the professional literature and the daily conversation of parents and teachers for years. These average ages are just that and nothing more. They are merely approximations and are meant to serve as convenient indicators of developmentally appropriate behaviors in the normally from the individual. It is common to see deviations from the mean of as much as 6 months to 1 year or more in the appearance of numerous movement skills The tendency to exhibit individual differences is closely linked to the principle of readiness, and it helps to explain why some individuals are ready to learn new skills when others are not.