It's not!! We were told as kids that prophets in general became prophets at that age around 40!! It's the case for Mohammed at least and if your read up on his life around that age, you can easily imagine he was going through a mid-life crisis!!
One can imagine anything, including unrealistic and baseless things. Reality, however, is different. This is evident by the many people who carefully studied his biography and ended up embracing Islam (some of them being heavily anti-Islam themselves before that).
A mid-life crisis is not necessarily a bad thing!! It's probably one of the most important spiritual experiences a person can have. We are told that Mohammed used to spend long times in the Hiraa cave kinda meditating or thinking about things that are not part of ordinary life.. Somebody with midlife crisis is one who has questions and searching for answers! They are asking: "Who am I? What am I doing in this world!?"
