From wikipedia:
Midlife crisis is a term coined in 1965 by
Elliott
Jaques and used in Western
societies to describe a period of dramatic
self-doubt that is felt by some individuals in the "middle years"
of life, as a result of sensing the passing of youth and the
imminence of old age. Sometimes, transitions experienced in these
years, such as aging in general, extramarital affairs, andropause, menopause, the death of
parents, or children leaving home, can
trigger such a crisis. The result may be a desire to make
significant changes in core aspects of day to day life or
situation, such as in career, marriage, or romantic
relationships.