Mine came on big time when I was around 19 (3 years ago now) after a period of a few months that included: - a lot of stress (good stress mostly, but still lots of stress/changes) - a round of antibiotics (which I'd never had before in my life) - going on the pill (not really sure if this was related or not)
But looking back I'm pretty certain I've had it my whole life to a lesser degree. The antibiotics and stress probably just triggered it into hyper-mode