Little is known about the effects of aging on the symptoms of bipolar disorder; whether episodes get more or less frequent, more or less severe and whether depression or mania tends to predominate. Researchers from the University of Iowa followed 148 people with bipolar disorder over a 20-year period. They found that those people who were under 44 at the start of the study had significantly more depressive symptoms as they aged. The earlier people's bipolar symptoms started the more problems they had with depression. The ratio between people's episodes of mania and depression remained fairly constant over the course of the study.
Coryell, W. ... [et al] - Age transitions in the course of bipolar 1 disorder Psychological Medicine August 2009, 39(8), 1247-1252