Puberty

In 2015, about 3 million teens had at least one major depressive episode. More than 2 million report experiencing depression that impairs their daily function. About 30% of girls and 20% of boys--totaling 6.3 million teens--have had an anxiety disorder, according to data from the National Institute of Mental Health.
Experts suspect that these statistics are on the low end of what's really happening, since many people do not seek help for anxiety and depression. A 2015 report from the Child Mind Institute found that only about 20% of young people with a diagnosable anxiety disorder get treatment. It's also hard to quantify behaviors related to depression and anxiety, like nonsuicidal self-harm, because they are deliberately secretive.
Still, the number of distressed young people is on the rise. Teen minds have always craved stimulation, and their emotional reactions are by nature urgent and sometimes debilitating. The biggest variable, then, is the climate in which teens navigate this stage of development.