Child psychiatrists typically begin seeing patients at 4 years old. In general, this reflects that:
Many behavioral challenges declare themselves when a child needs to leave their guardian's home for child care or educational reasons
Many psychiatric medications are tested for safety starting at 4 years old
Behavioral interventions are available earlier for children younger than 4 years old. Many therapy modalities, like PCIT and TF-CBT, focus on providing ways to navigate childhood behaviors while also providing psychoeducation and skills development for the guardian. However, if a guardian wishes to discuss medication management for a child younger than 4 years old, ideally a child psychiatrist will walk them through the pros and cons of available medication options, as well as what data, if any, is known for children under 4 years old. On the other end of the age range, child psychiatrists typically stop seeing patients after 18 years of age. However, in my opinion, there is no magical medical or psychiatric change between a 17 year old who needs a child psychiatrist and a legal adult; Rather, this age cutoff is more of a social and logistical construct than a hard and fast rule. For context, there are approximately 8300 practicing child psychiatrists in the United States. So in a country of about 330 million people, there are 8300 child psychiatrists but about 15 million children are believed to be in need of a child psychiatrist. To provide a more a more local example, Los Angeles is considered to have a high population of child psychiatrists. But in raw numbers, it's about 300 child psychiatrists for 3 million resident children. As a result, as children age there is a systemic pressure to transition them to an adult psychiatrist to make clinical space for younger children. Additionally, sometimes insurance will no longer cover the services of a child psychiatrist but will cover care with an adult psychiatrist. However, if the child's psychiatrist is willing to continue seeing the patient, subspecialty care with a child psychiatrist can continue.