Dr Kathleen Merikangas from the National Institute of Mental Health in the U.S. has been leading a team of researchers looking into the mental health of children in the U.S. The team carried out face-to-face interviews with 10,123 children aged between 13 and 18. They found that 22.2% of them suffered from a mental-health problem severe enough to cause them distress in their day-to-day lives. Anxiety disorders were the most common problem, being found in 31.9% of the children with a mental-health problem, followed by behaviour disorders (19.1%), mood disorders (14.3%) and drug problems (11.4%). 40% of the participants with mental-health problems suffered from more than one disorder.
You can find out more about this study here