I had heard from a psychiatrist that one of the reasons that so many kids are being diagnosis'd with BiPolar (BP) today is because they are looking at kids for BiPolar (BP) symptoms and recognized that they don't necessarily present the same as the diagnosis manual requires for adults, however, all the kids exhibiting these symptoms might not have true bipolar. As they mature physically hormones and brain chemicals start to balance out after puberty and as they mature emotionally and psychologically, they are able to cope with things better and ward off many depressive and explosive episodes. When these things happen, they no longer exhibit signs that appear to be BiPolar (BP). I think if that happens, they did not outgrow BiPolar (BP), they never had it to begin with.
ETA: As an after thought, no matter what it's labeled as, if it stops or even gets a lot better someday, that can only be a great thing!