As the "young'un" among people on this thread, let me venture out with an opinion.
I think the "mid-life crisis" is a result of running out of pre-determined purpose in life, i.e. running out of things that you're "supposed" to be doing.
When you're young, the next steps of your life are typically pretty obvious and well-defined. When you're a child, you're taught to go to school and get good marks so you can go to college. So you put a lot of zeal into that kind of stuff. Then when you get to college, you focus your energy into getting to the next step...getting good marks, a degree, and getting a job in the business world. Then once you get into business and start making money, you're going for a few big promotions. Then once you get those, you're supposed to start putting your energy into finding a mate and getting married. Then getting a house. Then having kids. Then raising those kids.
The big point is...for the vast majority of people, there's no "next step". The educational challenge built toward the professional challenge, the professional challenge built toward the self-reliance challenge, the self-reliance challenge built toward the family challenge. Then you reproduce and you start to think, "what was THIS challenge building toward??"
There's really no clear answer to that question for most people. It's around that age, around the 40s, when the kids are grown up enough to not require constant supervision, that a man starts asking himself, "what is the POINT of my life??"
It's almost like some of the free-roaming video games that we grow up playing these days...after you beat the "final boss", the game kind of dumps you off back into its 'world', but without a lack of clearly defined objectives. The idea is that you can still explore all of the mini-games and continue to upgrade your character and discover new things, but since there is no clear objective you're building toward, it's all kind of "hollow".
Some people respond to this time in life with a "mid-life crisis", when they finally decide to try and do all the things they wanted to do when they were young, now that they have the financial means to do so. Thus, the sports car, the adventures, the young women.
Some people respond by finding ways to "keep busy". My folks seem to be running in an endless cycle of fixing up their house. Once one room is fixed up, they move to the next one, and the next one, and so on until they're all done...by then the first room is "worn" enough that it's time for another renovation.
Some people just settle down and rot. Almost as if the "next challenge" is preparing to die.
Some people embrace faith and religion and start looking to the afterlife as a "next challenge".
I'm going to be 31 in about a week, and I feel like I've been in something of a "mid-life crisis" since maybe 27. I know you old codgers will laugh at that, but as a result of not buying into the whole girlfriend/marriage/kids thing, I've hit the "what is this life about" crisis a little early.
Don't get me wrong, I'd still like to experience all that if I can find a woman I can really "click" with, but I've had a lot of time to think, and because I got started late in the "dating game", I didn't have the huge libido driving me to hasty decisions regarding marriage and relationships. I saw the people around me rushing into that kind of crap simply for lack of anything better to do and decided, "Nah, that's not for me. Not unless I REALLY like spending time with someone". I thought I had found "someone" a couple of times, but nothing ever came of it...they could tell I wasn't buying into it hook/line/sinker and the lack of blind commitment to "the program" bothered them.
So now that the family-path has become an optional activity, rather than a mandate, in my mind, I'm at the same kind of place in my life.
I could look at a "career move", sure. I mean, there's still upward mobility where I work, but I've been looking ahead and seeing how the people in positions above me suffer emotionally and mentally because of their jobs, all for what are really modest increases in pay. I make more than a LOT of people in life at this point and, while there's still room for advancement, the pay increases here are modest and the job satisfaction paltry for the amount of life stress you take on.
So I'm left asking myself the same question...is what's left of my life just a long period of preparation and/or time-killing until I eventually die? And if so, what was all that nonsense before-hand building to? The schooling? The job?
Some people find purpose in helping other people...I see that as the "blind leading the blind". Everyone seems to suffer from the same lack of a sense of purpose...and those who give to others seem to be only facilitating others' arrival at a lack of purpose one day, or their ability to ignore it looming over them.
I can't say the way I've responded to my so-called "crisis" is healthy...I've taken my time to watch how other people deal with it, and I find they go out of their ways to remain ignorant, or seek out a "program" created by someone else, propping up something trivial as "greater than themselves".
I've reached a point where the entire human race disgusts me...they're all morons running around doing all they can to refuse to face the question of "purpose", refuse to question their own existence or to accept that maybe their paltry achievements DON'T make them worth more than any other lifeform.
Human beings are a disease, a cancer of this planet. You're a plague.
And what makes me sickest of all is the unavoidable fact that I'm one of them...and that despite all of my insight and knowledge as a sentient creature on this globe, I don't have an answer either. I can't seem to figure out what it is I'm supposed to be doing here, what's "important" in this world, what I've been building myself to all this life, and what, most importantly, is worth dying for, worth wasting the precious remaining 40-odd years of my life doing...or worth sacrificing those years to accomplish.
LOL...how can I ever get married or have children?? I hate human beings...and they want me to bring MORE of them into this world?? I can't even pretend that mine would be somehow "better" than the rest, because I can't even evolve beyond the filth that is humanity MYSELF.
I need to just move to a shack in the wilderness and start mailing bombs to people.