The definition and history of the midlife crisis.

Midlife crisis: A period of personal emotional turmoil and coping challenges that some people encounter when they reach middle age, accompanied by a desire for change in their lives, brought on by fears and anxieties about growing older. The phrase "midlife crisis" was introduced in 1965 by the psychoanalyst and social scientist Elliott Jaques in a study of creative geniuses. In the lives of numerous composers and artists, Dr. Jaques found abrupt changes in style or declines in productivity about age 35. A midlife crisis is experienced by many people during the midlife transition when they realize that life may be more than halfway over. Sometimes, a crisis can be triggered by transitions experienced in these years, such as andropause or menopause, the death of parents or other causes of grief, unemployment or underemployment, realizing that a job or career is hated but not knowing how else to earn an equivalent living, or children leaving home. People may reassess their achievements in terms of their dreams. The result may be a desire to make significant changes in core aspects of day-to-day life or situation, such as in career, work-life balance, marriage, romantic relationships, large expenditures, or physical appearance(Gordon, 2008). Academic research since the 1980s rejects the notion of midlife crisis as a phase that most adults go through. In one study, fewer than 10% of people in the United States had psychological crises due to their age or aging. Personality type and a history of psychological crisis are believed to predispose some people to this "traditional" midlife crisis. People going through this suffer a variety of symptoms and exhibit a disparate range of behaviors.

Reference
http://en.wikipedia.org/wiki/Midlife_crisis
Gordon, L. A. (2008).Midcareer Malaise. ABA Journal, 94(9), 38-43.