Separation anxiety in babies and young children kicks in about 6-8 months of age and stays in place until well over 5, sometimes for much longer. The separation distress system is located in the lower brain and in early life it is genetically programmed to be hypersensitive, for basic survival purposes. This distress is much the same as a physical pain.
While anxiety is a normal human experience, when children are suffering genuine separation distress we need to take it seriously. I share in these resources many ways of soothing distressed children, helping them feel connected to you even when you are apart, and how to trigger feel-good chemicals in their brain so that they can feel safe.