Yes, separation anxiety disorder (SAD) is considered a mental illness based on its inclusion in the DSM-5. The DSM-5, or The Diagnostic and Statistical Manual of Mental Disorders, is the guide used by mental health professionals to diagnose mental health conditions.
The DSM-5 defines separation anxiety disorder as developmentally inappropriate and extreme anxiety concerning separation from those to whom the individual is attached, as evidenced by at least three (3) of a battery of behaviors ranging from severe reluctance to leave home to repeatedly having nightmares about being separated. The anxiety must be persistent, lasting at least 4 weeks in children and adolescents and 6 months or more in adults.
This 3-minute, online quiz can be used if you think your child is having emotional, attentional, or behavioral difficulties.