Middle adulthood (or midlife) refers to the period of the lifespan between young adulthood and old age.  This period lasts from 20 to 40 years depending on how these stages, ages, and tasks are culturally defined.  The most common age definition is from 40 to 65, but there can be a range of up to 10 years on either side of these numbers.  For the purpose of this text and this chapter, we will define middle adulthood from age 40 to 65.  Research on this period of life is relatively new and many aspects of midlife are still being explored.  This may be the least studied period of the lifespan.  Midlife is a central, pivotal period in the life course. It falls at a critical juncture examining changes that go on physically, cognitively, and socially. 

Midlife has a somewhat unique advantage in the life course with the juxtaposition of gains and losses for aspects of physical, cognitive, and psychosocial changes that go on. We will identify social benefits and complexities in middle adulthood in addition to identifying aspects of decline in cognitive and physical functions. And this is a varied group.  We can see considerable differences in individuals within this developmental stage.  There is much to learn about this group.  In the United States, the large Baby Boom cohort (those born between 1946 and 1964) are now midlife adults and this has led to increased interest in this developmental stage.