This chapter examines how age matters to individuals and groups, and to social life and the organization of the life course. It illustrates how age is used to categorize and judge oneself and others, and how age is used to create distinct life phases and allocate social roles and activities in education, work, retirement, and family. This structure is reinforced through laws and policies, the design of institutions and physical environments, and social expectations. This structure can result in ageism, age segregation, and age conflict. Age must be understood in connection to gender, race, and social class. Differences among older people may not only reflect their unique experiences but also social processes that perpetuate inequality. Cohorts create new patterns of aging and the life course based on their unique composition and historical location. One can better anticipate the future of aging by studying cohorts who are now young.