Vexing certainty

When does imagination in individual humans begin the steady decline toward a state of inactivity? I wonder at what age the human mind becomes so rutted in certainty that imagination seizes and the rigid brain keeps the wondering mind at bay? Does chronological age (or maybe lack of new thoughts?) make the brain no longer "flexible" to new routes, to new circuitry? Is there a "mechanical" component to it? Or at a certain point in one's life does "der kopf" overflow? Not with knowledge, but with certainty? Of course when we agree with the thoughts of someone, we are pleased when that someone expresses these thoughts with a degree of certainty. But when we disagree with the thoughts of someone, we think them inflexible when they hold to their certainty. Vexing, isn't it?