These stages are approximate. The age isn't as important as the progression.
Some children reach a stage faster, some reach it slower.
Generally, all kids go through much the same things. Knowing what to expect can help parents deal with the frustrations and problems that are quite normal for each age. Behaviors that get stuck in one stage are cause for concern, otherwise, don't worry. One can get by parenting easy kids with common sense and love. Parenting difficult kids requires love, compassion, endurance, and skill that is not common.