Mid-life Crisis

From: How we Develop our Personality Types
Quote: An age-old question which most people have pondered at some point in their lives is "why are we the way we are"? Unfortunately, I can't clue you in on the answer to that question in a really authoritative way, but I can tell you about work that has been done to determine how people's Personality Types are developed.

From 35 - 50 years
We pay attention to our fourth, inferior function. We feel a need to develop it and use it more effectively. Our INFJ begins to use his Extraverted Sensing function. He becomes more aware of his surroundings and begins to take in information from others in a more literal, practical sense. He continues to rely on his dominant Introverted iNtuitive function to take in information, but he is more able to use his Extraverted Sensing function than he has been before in his life. Some researchers have attested that the appearance of our inferior functions at this phase of life may be responsible for what we commonly call the "mid-life crisis".
http://personalitycafe.com/general-psychology/1037-how-personality-types-develop.html