“Is ADHD a neurological disorder or a behavioral disorder?”

I would call it a neuropsychiatric disorder that is sen by psychiatrists and neurologists. Also by family physicians when it is easy to diagnose. It is definitely a cognitive problem associated with abnormal wiring (white matter tracks) in the brain. Often patients grow out of it by adulthood or it is no longer a problem if the patient does have an occupation that requires a lot of thinking!

It is both

It is both and it can improve with appropriate care.