Developmental age is a reference to how closely a person's physical and mental development parallels with normal developmental milestones (such as walking, talking, etc.). The age at which these "milestones" are due to be reached is an average of the age at which research subjects achieved these activities. The vast majority of children reach these milestones close to these proposed dates. Achieving some of these milestones early can be a sign of "giftedness" while delays in achieving the milestones can indicate some degree of physical or mental retardation.
Developmental delays can include both mental and physical delays that could be caused by physiological problems or by environmental difficulties such as premature birth, inadequate nutrition, lack of physical or intellectual stimulation, or lack of appropriate medical care.
For example, a feral child might be chronologically eight years old but have the behaviors and language skills of a three year old child. Therefore, their developmental age would be three years old.