I am a little confused by the term "cognitive changes". The above posters all described things which would most likely change a persons perspective but I would not call them cognitive changes. I would think cognitive changes may occur from illness such as brain tumor or a traumatic brain injury. Also Alzheimer's would cause a cognitive change, although hopefully not at "middle age".
In the United States, most people who reach middle age have been married and probably divorced. Divorce changes one's perspective profoundly. Many middle-aged people have children who are now adults or are nearing adulthood; grandchildren may be part of the family picture. Becoming a grandparent is another life-changing event that places one's past, present, and future in a new light.
Reaching middle age, if one has been fortunate enough to remain employed by the same company, often places a person in the downsizing/layoff crosshairs. It's cheaper to hire two new young employees than it is to keep one longtimer who has reached the top of the salary scale.
I would think any major change would exacerbate this issue. For instance, any of the following would definitely cause a change in the way people see the world and think about themselves and their surroundings: loss of a job (which in today's economy is VERY common), death of a spouse or child, divorce, beginning a new job, getting out of the military (which is for many people a serious culture shock which takes time and sometimes training/counseling to adjust to), going back to school, dating after a divorce or death of a spouse, major illness (cancer, swine flu, AIDS, etc.).
I would say that one of the most profound changes to take place during middle adulthood is the idea of being cognizant of a perceived finality in consciousness. This construction of a “mid life crisis” usually happens at a stage which is, not surprisingly, at the “mid way point” of life. It is when people begin to recognize that there is some end that can be perceived. This might manifest itself in a variety of ways. Changes to one’s life and what one does could be present. Sometimes, an overhaul of how one lives that becomes a very big point of change in work and family could also be present. The individual narratives speak clearly to this, but job changes and family changes for both better and worse can be present at such a stage of life. Part of this might arise from the perception that what was done has to be altered in the face of what needs to be done with an understood end in sight. This impacts one’s cognitive framework as everything in consciousness becomes filtered through this “need to change.”