Middle age is the period of age beyond young adulthood but before the onset of old age. Various attempts have been made to define this age, which is around the third quarter of the average life span of human beings.
According to Collins Dictionary, this is "... usually considered to occur approximately between the ages of 40 and 60". The current edition of the Oxford English Dictionary gives a similar definition but with a shorter span: "The period of life between young adulthood and old age, now usually regarded as between about forty-five and sixty." The US Census lists middle age as including both the age categories 35 to 44 and 45 to 54, while prominent psychologist Erik Erikson saw it ending a little later and defines middle adulthood as between 40 and 65. The Diagnostic and Statistical Manual of Mental Disorders, the standard diagnostic manual of the American Psychiatric Association, used to define middle age as 40–60, but as of Edition IV (1994) revised the definition upwards to 45–65.
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Middle-aged adults often show visible signs of aging such as loss of skin elasticity and graying of the hair. Physical fitness usually wanes, with a 5–10 kg (10–20 lb) accumulation of body fat, reduction in aerobic performance and a decrease in maximal heart rate. Strength and flexibility also decrease throughout middle age. However, people age at different rates and there can be significant differences between individuals of the same age.
Both male and female fertility declines with advancing age. Advanced maternal age increases the risk of a child being born with some disorders such as Down syndrome. Advanced paternal age significantly increases the risk of miscarriage, as well as possibly slightly increasing the risk of Down syndrome, schizophrenia, autism, decreased intellectual capacity, and bipolar disorder. Most women will experience menopause, which ends natural fertility, in their late 40s or early 50s. But fertility in these women already started to decline in their 20s, and continues to decline every year. Ten years before menopause, their fertility is already very low.
In developed countries, yearly mortality begins to increase more noticeably from age 40 onwards, mainly due to age-related health problems such as heart disease and cancer. However, the majority of middle-age people in industrialized nations can expect to live into old age. Life expectancy in developing countries is much lower and the risk of death at all ages is higher.
See also 
- Collins Concise Dictionary 4th edition 1999 Collins (an imprint of Harper Collins Publishers) Glasgow ISBN 0-00-472257-4
- AskOxford: middle age
- middle age. CollinsDictionary.com. "Collins English Dictionary" - Complete & Unabridged 11th Edition. Retrieved December 05, 2012.
- Shephard, Roy J. (7 March 1998). "Aging and Exercise". Encyclopedia of Sports Medicine and Science (T.D.Fahey). Retrieved 2007-06-26.
- Rabin, Roni (2007-02-27). "It Seems the Fertility Clock Ticks for Men, Too". The New York Times. Retrieved 2009-10-16.
- Fertility Treatment Less Successful After 35: In Vitro Fertilization Doesn't Compensate for Decreased Fertility With Age
- Heubeck, MA, Elizabeth; Reviewed By Brunilda Nazario, MD (2005-06-29). "Age Raises Infertility Risk in Men, Too: Risks associated with men's biological clocks may be similar to women's.". WebMD. MedicineNet. Retrieved 2009-10-16.
- "Miscarriage significantly associated with increasing paternal age". Columbia University's Mailman School of Public Health. 2006-08-03. Retrieved 2009-10-16.
- Raeburn, Paul (February/March 2009). "The Father Factor: Could becoming a father after age 40 raise the risks that your children will have a mental illness? (PDF)". Scientific American Mind: 30–33. Retrieved 2009-10-16.
- Cannon, Mary (2009-03-10). "Contrasting Effects of Maternal and Paternal Age on Offspring Intelligence". Public Library of Science. Retrieved 2009-10-16.
- BBC - Health: Menopause
- Cannon, Mary (2003-06-26). "Male Biological Clock is Ticking, Too". WebMD Health News. Retrieved 2009-10-16.
- "Life Expectancy Profiles". BBC. 6 June 2005. Retrieved 2007-06-26.
- "UK cancer mortality statistics by age". Cancer Research UK. May 2007. Retrieved 2007-06-26.
- The Third Age
- Does Age Quash Our Spirit of Adventure?, a segment on NPR's All Things Considered on an aging study done by middle-age neuroscientist Robert Sapolsky
- Ali Khan, Living Fully at Forty and Beyond
|Stages of human development