I think of middle-age as the period of life that begins at the end of the mid-life crisis. This is not to deny the mid-life crisis, which is very real and occurs in one's middle years. But because my focus is the benefit of these years, I begin when the crisis has passed. (An analogy: one might think of the beginning of the Cenozoic era not as the time the dinosaurs were killed off, but rather slightly afterwards, when the mammals began to take over the planet. Putting a happy face on extinction.)
For most people (in many cultures, "developed" and undeveloped, around the world) the sense of happiness or well-being is high in youth and high late in life, but plunges in the middle, usually at about age 50. If charted over a lifetime, this phenomenon makes a "U" shape, high on the ends, low and flat (because it goes on for a number of years) in the center. It is during the low flat time that people often despair, feeling their lives have no meaning, detecting nothing to look forward go, and leading some to get tattoos, buy little red sports cars, or have affairs with maids or gardeners.
It has never been clear why this crisis occurs, especially since cultures around the world pose such different demands on people as they age. It can't be pinned down to children leaving home, or frustration at the stalling of professional ambition, and it seems immune to any kind of counseling.
A new scientific finding may help illustrate why. Researchers assembled observations of more than 500 chimpanzees and orangutans in zoos, sanctuaries, and research centers around the world, controlling for genetics, physiology, social pressures, and stress factors. What they found was quite amazing: When it comes to mid-life crisis, we are not alone. Apes have a similar U-shaped dip (or "lull in happiness") in the middle of their lives. Their life expectancy is roughly 50 years, and the dip is at about age 30.
This means that we should stop struggling to pin midlife disenchantment on our lots or on each other. As one scientist said, for fundamental biological reasons (though the exact mechanism is not known) a midlife crisis is "completely normal," and "apparently out of your control."
What is the practical value of this information? Perhaps those who have not yet reached it can think of it as they would a menstrual period: it's coming, it may well be unpleasant, during it the world will seem inhabited by oppressive clueless assholes who strip away all possibility of joy. But if you can take a longer view, you might see glimpses of life on the far side, when all will be back to normal. Or better.