Worried About a Moody Teen? - Wall Street Journal Mental Illness Often Starts in Adolescence. Telling Typical Angst From Serious Problems Everyone warns parents about the drama of the teen yearsthe self-righteous tears, slamming doors, inexplicable fashion choices, appalling romances. But what happens when typical teen angst starts to look like something much darker and more troubling? How can parents tell if a moody teenager is simply normalor is spinning out of control? This may be one of the most difficult dilemmas parents will ever face. Studies show that about 20% of teenagers have a psychiatric illness with depression, anxiety and attention-deficit hyperactivity disorder being among the most prevalent. Yet parents of teens are often blind-sided by a child's mental illness. Some are unaware that mental illnesses typically appear for the first time during adolescence. Or they may confuse the symptoms of an actual disorder with more normal teen moodiness or anxiety.