In investigations of decline of intellectual status with age, cross-sectional and longitudinal studies give divergent results--the former show almost universal declines in test performances among older groups while the latter often show gains. Although few structure-of-intellect (SI) factors have been explored in relation to adult ages, two kinds of SI abilities stand out with respect to defects in the elderly--the formation and use of systems of information and the ability to be flexible in shifting between classes and in transformation of information. The older person's capacity for information intake and for handling information in productive thinking may be limited, perhaps due to a weakness in short-term memory. A rather general weakness is susceptibility to interference and inability to correct errors. Little is known about the causes of intellectual decline. Heredity appears to have a bearing, possibly largely because it affects development. Inadequate oxygen supply to the brain is a suspected cause of decline. Exercise of brain cells through continued education, formal and informal, including that provided by occupational activity, may have halting effects, but this hypothesis needs better experimental investigation. (aj)