Consider the following statistics: A 45-year-old American woman who remains free of heart disease and cancer can expect to see her 92nd birthday; a 45-year-old man in similar condition, his 88th birthday (Arias, 2014; Mensah & Brown, 2007). This means that today's 45-year-olds who maintain reasonably good health can look forward to living another half of their lives. Throughout much of human history, 40 was regarded as a fairly ripe old age. Life expectancy at birth in the United States in 1880 was only 39.4 years (Jaminet & Jaminet, 2012). But with extraordinary advances in biomedicine in the last century, longevity has become a global reality. A profound shift has taken place in the human condition, and a new developmental stage – a second stage of adulthood – has been added to the life cycle (Bateson, 2010). 'Imagine the day you turn 45 as the infancy of another life', writes Gail Sheehy (1995), author of New Passages: Mapping Your Life Across Time. Millions of people in their forties, fifties and sixties today, Sheehy notes, are able to make dramatic changes in their lives and habits, and look forward to living decades more in relatively good health. She asserts that the traditional boundaries of adulthood, beginning at 21 and ending at 65, are completely out of date and in need of revision.

Many contemporary scholars of adult development agree. They point out that the stage between the end of young adulthood and the beginning of true old age – which these days amounts to a period that can last a half century – is a time for new possibilities and discoveries, when people's personalities and brains are still pliable and open to change (Cohen, 2005). The decades after age 45 are motivating more and more people to 'stretch their independence, learn new skills, return to school, plunge into new careers, rediscover the creativity and adventurousness of their youths, and, at last, to listen to their own needs' (Sheehy, 1995: 140).