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 sees it ending a little later and defines middle adulthood as between 40 and 65. (http://en.wikipedia.org/wiki/Middle_age)