Bipolar disorder usually strikes between the ages of 15 and 25, and is extremely rare in preteens, according to a major study: Age at onset versus family history and clinical outcomes in 1,665 international bipolar-I disorder patients
The findings are old hat. It's long been known that manic-depression most often begins around the age of 20, give or take a few years. Onset in later life is less common while earlier onset is very unusual.
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