Middle age is the period of life 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 OED gives a similar definition but with a later start point "... the period between youth and old age, about 45 to 60".
The US Census lists middle age as including both the age categories 35 to 44 and 45 to 54, whilst Erik Erikson sees it ending a little later and defines middle adulthood as between 40 and 65.
In many Western societies, this is seen to be the period of life in which a person is expected to have settled down in terms of their sense of identity and place in the world, be raising a family, and have established career stability. It is also a period often associated with the potential onset of mid-life crisis.
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 large differences between individuals of the same age. 
Female fertility declines significantly after age 40, and an advanced maternal age increases the risk of a child being born with some disorders such as Down’s Syndrome. Some conditions are also correlated with advanced paternal age. Most women go through the menopause, which ends natural fertility, in their late 40s or 50s. 
In developed countries, yearly mortality begins to increase from age 40 onwards, mainly due to age-related health problems such as heart disease and cancer.   However, the majority of middle-aged 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. 
The prime of life
Some people  challenge the concept that middle age is something to dread. They assert that with the right attitude and careful planning, middle age can be truly a person's best years. Those age-positive groups range from advocacy groups such as the American Association of Retired Persons (AARP) to purely social clubs like the Red Hat Society.
- 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
- Collins Concise Dictionary 4th edition 1999 Collins (an imprint of Harper Collins Publishers) Glasgow ISBN 0004722574
- Shephard, Roy J. (7 March 1998). "Aging and Exercise". Encyclopedia of Sports Medicine and Science. T.D.Fahey. Retrieved 2007-06-26.
- BBC medical notes
- "Life Expectancy Profiles". BBC. 6th June 2005. Retrieved 2007-06-26. Check date values in:
- "UK cancer mortality statistics by age". Cancer Research UK. May 2007. Retrieved 2007-06-26.
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