Development takes on new meaning in adulthood because the process is no longer defined by physical and cognitive growth spurts. Adulthood, which encompasses the majority of a person's life span, is marked instead by considerable psychosocial gains that are coupled with steady but slow physical decline.
Age clocks, or the internal sense of timing of physical and social events, determine the various life stages through which adults pass. Although people age at different rates, the majority of Americans, reinforced by social norms, pass through a series of predictable periods.
Perhaps the best‐known stage theory of adult development is that offered by Daniel Levinson . According to Levinson, the ages of 17 to 45 encompass early adulthood, which he divides into the novice phase (17–33) and the culminating phase (33–45). Levinson further divides the novice phase into the stages of early adult transition (17–22), entering the adult world (22–28), and age‐30 transition (28–33). The culminating phase (33–45) consists of the settling down (33–40) and midlife transition(40–45) stages. As with any stage theory, these stages are only a guide for the development that normally occurs along a continuum. Not everyone progresses through each stage at exactly the same age.
The young adult years are often referred to as the peak years. Young adults experience excellent health, vigor, and physical functioning. Young adults have not yet been subjected to age‐related physical deterioration, such as wrinkles, weakened body systems, and reduced lung and heart capacities. Their strength, coordination, reaction time, sensation (sight, hearing, taste, smell, touch), fine motor skills, and sexual response are at a maximum.
Additionally, both young men and women enjoy the benefits of society's emphasis on youthfulness. They typically look and feel attractive and sexually appealing. Young men may have healthy skin, all or most of their hair, and well‐defined muscles. Young women may have soft and supple skin, a small waistline, and toned legs, thighs, and buttocks. Early in adulthood, neither gender has truly suffered from any double standard of aging: mainly, the misconception that aging men are distinguished, but aging women are over the hill.
With good looks, great health, and plenty of energy, young adults dream and plan. Adults in their 20s and 30s set many goals that they intend to accomplish—from finishing graduate school, to getting married and raising children, to becoming a millionaire before age 30. Young adulthood is a time when nothing seems impossible; with the right attitude and enough persistence and energy, anything can be achieved.