Piaget's stages of cognitive development

Piaget's theory claims that children's cognitive development occures over four progressive stages. These are; sensorimotor, preoperational, concrete-operations, and Formal-operations. Without the sucessful completion of a stage the proceeding stage can not be reached. Therefore not all children complete the stages at the same age. As a teacher i must take this into account, that not every student will be at the same stage.