Mid-life blues is a cross-cultural phenomenon

There is no explanation given, various possible hypotheses are proposed, though suprisingly not the most obvious one that it is a response to the onset of clearer signs of aging, though there is mention of the adjustment to fuller understanding of ones limitations and 'infeasibilities'.

"It looks from the data like something happens deep inside humans. For the average person in the modern world, the dip in mental health and happiness comes on slowly, not suddenly in a single year. Only in their 50s do most people emerge from the low period. But encouragingly, by the time you are 70, if you are still physically fit then on average you are as happy and mentally healthy as a 20 year old. Perhaps realizing that such feelings are completely normal in midlife might even help individuals survive this phase better."