Brain cells do continue to grow throughout our lives if they are appropriately nourished. Maturity is a state few people attain. Many of our beliefs about the aging process are now being shown to be inaccurate as we discover species in the animal kingdoms which have a totally different physiology. Quantum physics is revealing that our universe is a far more fluid and fickle construction than any of us would care to imagine. At those quantum levels of subtlety, our expectations and actions determine the outcome - not a fixed 'rule' of physics.
Thoughts become self-fulfilling prophecies.
In addition, we selectively view the situation so that it fits with our preconceived views, our paradigms. Conversely, when we go into a situation expecting the best, then we set the scene for the best to happen. In addition, we selectively view the situation looking for evidence of benefits and opportunities for advancement. Misunderstanding what the change entails, Disagreement about the benefits of change, and, Concern they may not have the skills and ability to cope in a new world. Fear of Loss People often resist change because they think they will lose something of value.