The stage concept implies that the course of development is divided into step-wise levels, with clear-cut changes in behavior from one stage to the next.
As a developmental concept, a stage usually defines a set of behaviors that occur together. As a group, they characterize a quality of behavior that differs appreciably from the quality of behavior in earlier and later stages.
Stages follow each other in an orderly sequence, and the transition from one stage to the next usually involves a process of integration, whereby the behavior from the earlier stage is transformed into the next, along with some new elements. While environmental factors may speed up or slow down development, they do not change its sequence.