Have you always been bipolar?

I've always wondered if bipolar is something that's always been with you but remains undiagnosed until your first manic episode, or if bipolar disorder is something you have once you have your first manic episode. What do you think? I myself don't know because you may say there were signs before (I would have periods of high productivity and then lethargy and feel blue) but they seemed like normal mood fluctuations. I've also always been extremely sensitive which I don't know if it's related to bipolar.