Of course, in developmental psychology as in other branches of psychology we rely on theories, which are integrated, organized sets of statements that try to describe, explain, and predict behavior. There were originally two basic theoretical views of the developing person. If this sounds a lot like the nature versus nurture controversy, I myself find a resonance. Another spat within the theoretical community has been whether development is continuous or discontinuous. Continuous theories do not break up development into stages.
Discontinuous theories emphasize a particular skill or attribute at a particular age.
Age-stage theories always emphasize that the stages are qualitatively different from each other and represent a definite break from the previous way of thinking or behaving. Some theories emphasize the stability of behaviors or abilities across a person's life. These theories typically stress heredity or early experience. Other theories state that change is possible and likely if new experiences support it. There is a lot of controversy on the stability versus change issue, especially across different domains of development.