Cross-sectional and longitudinal relationships among age, cognition, and processing speed.

Cross-sectional and longitudinal age effects on cognitive function were examined in 302 older adults followed longitudinally. Processing speed was related to cognitive performance at cross-section, and change in speed predicted within-person longitudinal cognitive decline. Statistical control of processing speed greatly reduced cross-sectional age effects but did not attenuate longitudinal aging effects. This difference in processing speed's ability to account for cross-sectional and longitudinal age effects is discussed in the context of theories of cognitive aging and methodological and statistical issues pertaining to the cross-sectional and longitudinal study of cognitive aging.