The present paper starts by introducing different perspectives of the aging process and includes biological, psychological, and sociological theories in its scope. The article addresses the issue of when "old age" begins and why a distinction is made between the third and fourth age. With increasing age, it becomes more and more difficult to differentiate between health-related losses due to illnesses or to aging. However, this can be important with respect to health behavior and health care. Having the best possible health in old age is an important factor for a good life in old age. Over their whole lives, from childhood to old age, people can actively contribute to their health in old age. But health is not the sole criterion for a good life in old age. Having interests and aims are just as important as being integrated in a social network. In old age, people often differ greatly and this is why there is such variety in what people consider to constitute a good life for themselves in old age.