Most people affected by bipolar disorder are those in their late teens to early adulthood (ages 18-25 roughly). The percentage of those who have bipolar late in life is considerably less, while the percentage of children is estimated to be from 0-3% due to debate over the diagnosis of pre-adolescents. Genetic factors can also come into play, as well as detrimental environmental factors.
Probably the most famous celebrity in the UK to have bipolar disorder is Stephen Fry. Stephen even made a TV programme about his personal experience and how it affected his life. There are likely to be many more who we do not know about as most cebraties choose to keep matters of this nature private.