It is not even sure that the beast exists. Women go through menopause between the ages of 42-48. The amount of the hormone estrogen decreases sharply, important parts of the reproductive system shrink and menstruation ceases. Men do experience a gradual decline in testosterone levels but nothing as sharp as the woman's deterioration of her estrogen supply. The lack of satisfaction with life, career, or spouse that sets in has to do with this gap of expectations.
People get more disappointed and disillusioned with age.
They have nothing to look forward to. They feel spent, bored, fatigued and trapped. Some adults make a transition. They define new goals, look for new partners, form new families, engage in new hobbies, change vocation and avocation alike, or relocate. They regenerate and reinvent themselves and the structures of their lives.