It was always my understanding that people are genetically predisposed to having bipolar disorder, but that the illness may not show itself until a certain age (maybe because of a trigger or maybe for no apparent reason at all). I know that in my case, though I did have a few bad episodes earlier in my life, my bipolar illness didn't become disabling until I was in my mid 30s. I know of many cases where people just have a couple (or even just one) major episode in their life, and then either no more or only mild ones. I think it varies by person and their situation, etc.

Though I was clearly genetically predisposed to having bipolar disorder, I don't think I "officially" had the disorder as a small child. My symptoms didn't really become apparent until I was about 14 years old, and even then the symptoms were regarded as other things (teenage strife, severe disappointment, etc, etc). Some of my elevated episode behavior was even almost regarded as personality traits.