This is really hard to answer because women go through menopause which is all about hormones that they can not control. It's an actual physical medical condition.
Men on the other hand go through something too, which is partly physical, but mostly in their mind.
I'm not answering this very well because it is all such a mystery, and it affects every person differently.
I'd say that women do go through the mid life crisis, but it's more complicated because there are so many different things that can cause it. For example: Hormones, a thyroid problem, age, ect ect. It's just more complicated and it is in no way the "same" as the male crisis.
Note:::: Before anyone starts attacking me, I know about these things because my wife and I are at the age and are experienceing all of them together. Don't anyone start telling me that I don't understand women because I do. Have been to the doctor with my wife and trust me, I'm living it. We are both going through what the old folks used to call "the change" :)
You're right about the decreasing levels, but it comes much later. Men start their crisis before that just because they "think" they're getting older.
Ooh, I'd always thought male midlife crisis was a result of decreasing levels of testosterone, and female midlife crisis was a result of menopause.
Which, now that I think of it, is definitely contributing to the "in their mind"