Could you "grow out" of having mental disorder?

It depends: Many disorders fluctuate spontaneously over time; there may be symptom-free periods (even for years). Some improve partially - e.g., some with adhd lose the hyperactivity, but retain concentration problems, as they age. Some personality disorders may improve over time. Some disorders stay more or less the same, and some get worse over time. Treatment, of course, greatly influences illness course.