With BD, I believe one has to be genetically predispositioned for it. Usually, a stressful situation or trauma triggers the bipolar gene. Some may never "become" bipolar, but may carry the genetic material and pass it on down the line. Most triggering occurs around the early 20's, as so as I've often read.

With me, I was born this way, or triggered at a very very young age because I remember cycling at 6, but didn't get DX'd till I was 45. Neither one of my parents was mentally ill (even though I strongly disagree).