As children grow and develop, their abilities change. To illustrate this, the American Red Cross has created a guide to show children’s physical, social and mental abilities at different ages. It will also help you as an au pair to plan safe and age appropriate activities.
Children typically go through certain states and reach different milestones at predictable ages. Sitting, walking and talking are all examples of milestones and skills learned. The milestones are typically reached during a certain time and they are learned in a certain order, for example a baby would learn to hold their head up before learning to sit up, and would learn to stand before learning to walk.
You can treat these as guidelines for what might be considered typical development. No two children are alike. Even children of the same age can show different behaviors or have different abilities.