This concept means that we must always keep the age of the child in mind and that we must not have unrealistic expectations. Our rules and responses must be consistent with the age level of the child and be appropriate and wise. Remembering this will help reduce parental frustration and help with teaching the child in a way that is easy for them to learn new concepts.
An example would be asking your two year old to make their bed or clear the dinner dishes from the table; they are too young to complete those tasks on their own. A different example would be paying your teenager $50 if they make their bed or clear the dinner dishes from the table; they are way more capable and should not be so highly rewarded for a task that is so simple for someone their age.