The Diagnostic and Statistical Manual of Mental Disorders (DSM)
defines anxiety as a cognitive disturbance that manifests as a series of physical and emotional responses to feeling threatened, or in danger. These responses trigger different physical reactions that range from sweat, crying, irregular heartbeats, high blood pressure, nervousness, depression, body aches and overall malaise.
The anxiety disorder that is recognized as universal among children and teens is separation anxiety disorder, and the scientific community has reached a conclusion that many adult manifestations of anxiety disorder are a residual of anxieties suffered as children. Keep in mind that, when you are a child you are entirely dependent on an adult, and any sign of danger would be a traumatic event.
Teenagers, specifically, suffer anxiety mainly from assumptions of danger, because of learned associations and symbolization. During the adolescent stage, hormone production makes the frontal lobe of the brain particularly sensitive, which is why teenagers are notorious for their emotional outbursts and intermittently irrational thinking.
Therefore, the conclusion is that, in relation to the teenage brain, anxiety is a psychologically-induced disturbance caused by a combination of hormone production, an over-stimulated frontal lobe, personality traits, learned associations, and symbolization. Since these behaviors surface as defense mechanisms when danger is perceived, teenagers will manifest their emotion in a myriad of different ways that reflect that there is something wrong going on. When anxiety is at the core of someone's overall behavior and begins to disturb other aspects of life, it is related to as anxiety disorder.