Traditionally, it has been thought that bipolar disorder emerges most typically in the early to mid 20s of a person’s life. It has been known since the time of Emil Kraepelin (circa 1921), however, that children can be affected by this illness. While adolescent onset (mid-to-late teens) is now recognized as common and similar to the presentation of adult bipolar disorder, pediatric onset bipolar disorder remains the subject of debate, and its presentation is somewhat different than adult bipolar disorder.
Why do some children get bipolar disorder?
We do have some idea why some people get bipolar disorder as adults, and others get it much younger. The effect is known as genetic anticipation, which occurs when certain genes accumulate in later generations. We know that bipolar youth are highly likely to have members of their family belonging to previous generations (parents, grandparents, aunts, uncles) with bipolar disorder. When those genes add up over successive generations, and the person has a LOT of the polymorphisms that cause bipolar disorder, they tend to get it a) younger and b) more severely. This also helps to explain why pediatric onset bipolar tends to be a clinically severe manifestation.
While adolescent-onset bipolar is not uncommon, bipolar disorder in children under the age of 12 remains rare. Nonetheless, it is important to diagnose. Studies have found that for every year of untreated illness, a child is 10% less likely to experience a resolution of symptoms for 2 months or longer — yet, an average of 10 years passes between symptom onset and treatment.

How does pediatric-onset differ from adult bipolar disorder?
Pediatric onset bipolar is commonly characterized by very rapid cycling. This is an uncommon phenotype (what the disorder “looks like”) in adults, but the majority of children with bipolar disorder are rapid cyclers. Chronic irritability is also common and is part of the reason pediatric bipolar disorder is so controversial. An episodic pattern of moods — whether the manias are dysphoric or euphoric — is arguably the hallmark of bipolar disorder; chronic irritability doesn’t seem to fit the bill, and in children who do not experience depression, an alternative diagnosis may be more appropriate. Nonetheless, irritability is a common symptom even among those children who seem to clearly meet the criteria for bipolar disorder.

What happens to children with bipolar disorder?
Barring tragedy, children with bipolar disorder grow up to be adults with bipolar disorder. They need to be maintained on medication for the rest of their lives, or they are prone to relapse, just like any bipolar patient.
In my anecdotal experience (my symptoms started before the age of 10) I still have rapid cycling and dysphoric mixed manias as an adult. I urge those who are doubtful that rapid cycling bipolar is “real” or that it is somehow less valid to consider that many adults with this phenotype first experienced bipolar symptoms as children.
Recommended reading about bipolar in children
If you want to learn more about bipolar disorder in kids, I recommend the book The Bipolar Child by Demitri Papolos. It is really informative and helped me to better understand myself, as someone who had pediatric onset. I sat down in a library and read it all within a couple of hours.
