New frontiers in the future of aging: from successful aging of the young old to the dilemmas of the fourth age.

We review research findings on the oldest old that demonstrate that the fourth age entails a level of biocultural incompleteness, vulnerability and unpredictability that is distinct from the positive views of the third age (young old). The oldest old are at the limits of their functional capacity and science and social policy are constrained in terms of intervention. New theoretical and practical endeavors are required to deal with the challenges of increased numbers of the oldest old and the associated prevalence of frailty and forms of psychological mortality (e.g., loss of identity, psychological autonomy and a sense of control). Investigation of the fourth age is a new and challenging interdisciplinary research territory. Future study and discussion should focus on the critical question of whether the continuing major investments into extending the life span into the fourth age actually reduce the opportunities of an increasing number of people to live and die in dignity.