Typically, separation anxiety peaks when an infant is 9 to 18 months old. It has usually faded by the time a child is about 2.5 years old. Doctors do not consider it a disorder when it occurs at this developmentally appropriate stage.
Doctors may diagnose separation anxiety disorder if an older child or teen develops several symptoms of the condition that last at least 4 weeks. These symptoms also need to be severe enough to cause the child or teen to miss school or other activities that take them away from an attachment figure, such as a parent.