63.  Some researchers have criticized Piaget for his discontinuous model of development and inaccuracy of cognitive milestones.  Contemporary studies have shown a model of development that is more continuous than Piaget's model of stages & ages, and others argue that children reach cognitive milestones earlier than Piaget states.  Studies show that children can play, walk, or develop language skills earlier or later than the norm, based on cultural background or their environments.  Piaget belives formal operational thought begins between the ages of 11 and 20.  Many development psychologists disagree with him suggesting a fifth stage of cognitive development, known as the postformal stage.  In conclusion, it appears that further research has proven his developmental model to be outdated.