In my experience the majority of well made bows that have broken, were older bows. My impression is that wood 'matures' or peaks in quality, in regards to bowmaking, like humans or animals, as they age.
When green, wood doesn't make a good bow and when old it has lost elasticity and begun to decay. We might be talking very small amounts that no one would even notice.
At the same time, my experience with everything else in life tells me that just because you might be able to take stave and flash dry it without it checking and have a bow in a week doesn't mean its at its "best", whatever that means. I'd rather slow dry the stave, but not over a year. I'd rather have a stave that was dried for 6 months than one that was "kiln" dried, or one that was sitting around for 5 years.