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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 45 to 64, 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.
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 mid 40s or early 50s. However, fertility in women starts to decline in the 20s. Ten years before menopause, fertility is already very low.
In developed countries, yearly mortality begins to increase more noticeably from age 41 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.
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