Research suggests that bipolar disorder worsens with age. There is a significant increase in depressive episodes as the patient ages, and the episodes tend to become more severe and frequent. The average age-of-onset for bipolar is around 25 years-old, but symptoms could appear in the teens.
The research indicates that the severity of the depressive episodes is higher for patients who had an earlier-age-of-onset than those whose illness started late. However, the persistence of manic and hypomanic symptoms is not dependent on the age of onset. This implies that the depressive symptoms increase in persistence over decades for patients who acquire the illness as young adults.
Patients who are under the age of 30 at the onset experience a higher frequency of depressive episodes during the third, fourth and fifth decades of their life. In contrast, people who had their first symptoms at 45 years or older are stable most of the time with isolated episodes of depression. Patients who have an early onset are more likely to have depression for 30% of the time while those who get it when they are older than 45 years-old experience depression 22% of the time. It seems that people who have bipolar disorder while the brain is developing have the severest manifestations of the disease in later life.