This article by Barbara Geller, M.D. et al, studies rates and ages of onset of DSM-IV syndromal and subsyndromal comorbidity in prepubertal and early adolescent bipolar disorder phenotype (PEA-BP) compared to ADHD disorder.

Researchers  discussed whether Bipolar Disorder exists in prepubertal children (2000). There was agreement that both Bipolar types exist in this population, as well as other phenotypic possibilities that do not meet DSM-IV criteria.