The 4th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), published by the American Psychiatric Association stipulates that there must be manic or hypomanic episodes, before one can make a diagnosis of bipolar disorder, regardless of the patient's age.
A manic episode is characterized by a distinct period of abnormally and persistently elevated, expansive, or irritable mood, lasting at least 1 week (or any duration if hospitalization is necessary). During the period of mood disturbance, 3 or more of the following symptoms will have persisted (4 if the mood is only irritable), and have been present to a significant degree, causing marked impairment in social, academic, or occupational functioning: inflated self-esteem or grandiosity, decreased need for sleep, increased or pressured speech, flight of ideas or racing thoughts, distractibility, increased activity (either socially, at school or work, or sexually) or agitation, and excessive involvement in pleasurable activities that have a high potential for painful consequences (e.g. shopping sprees, sexual indiscretions, or foolish business investments).
Hypomanic episodes are similar to manic episodes, but the symptoms only need to have been present for at least 4 days.
Available data suggest that the lifetime prevalence of bipolar disorders is about 0.5-1.5% of the general population, with the onset of the illness usually being in the teenage years onwards. However, bipolar disorder may be diagnosed in younger individuals such as those you refer to, with a great deal of caution, provided they meet the diagnostic criteria mentioned above. One would take into consideration the presence of symptoms (such as irritability, or mood lability) that significantly exceeds that expected for the child's developmental stage. Bipolar disorder in children may be treated with medications, under the care of a physician with expertise in child psychiatry.