Bipolar disorder is presenting at a younger age of onset. In the past, many children with bipolar disorder were undiagnosed and ended up in juvenile detention centers. These children were also often misdiagnosed with severe attention-deficit/hyperactivity disorder (ADHD) or conduct disorder. In addition, an increased prevalence rate of substance use disorders and possibly stimulant prescriptions may be responsible for the shift in age of onset of bipolar disorder. The most compelling epidemiological observations have come out of the Netherlands, where they typically treat ADHD behaviorally rather than with stimulants. Childhood bipolar disorder remains unidentified in the Netherlands despite aggressive searching, whereas adolescent and adult bipolar disorder has been found in numbers similar to those in the United States. This provides cross-sectional epidemiological evidence that stimulants may be shifting the age of onset in patients who are presenting with potentially prodromal bipolar symptoms. Lastly, several researchers speculate that trinucleotide repeats in the DNA (deoxyribonucleic acid) may be responsible for so-called genetic anticipation phenomenon, by which expansion of trinucleotide repeats in successive generations may be shifting the age of onset to an earlier age. These are just some of the mechanistic possibilities for why we are seeing a shift in the age of onset of bipolar disorder.

Lewinsohn and colleagues found that the lifetime prevalence of adolescent bipolar I and II disorder is ~1%, with a 5% lifetime prevalence rate of subsyndromal bipolar disorder. Many symptomatic children and adolescents do not meet full duration criteria for a hypomanic or manic episode, or they may be one symptom short of meeting criteria for the disorder.