Inspection Time (IT) is a psychophysical speed measure that has been linked to a range of cognitive abilities with results finding that shorter IT is associated with superior performance in cognitive abilities. Following a recent suggestion by Nettelbeck and Wilson [Nettelbeck, T., & Wilson, C. (2004). The Flynn effect: Smarter not faster. "Intelligence," 32, 85-93.] that IT might have promise as a biomarker for cognitive decline during old age, this study tested the predictive validity of IT (both absolute estimate and change through time) for future cognitive performance and cognitive decline. Elderly participants (N = 124) were assessed on predictors IT and alternative common biomarkers, and outcomes fluid reasoning, crystallised ability, perceptual speed and working memory, on three occasions over 18-months. Results confirmed that IT predicted future performance on fluid reasoning, perceptual speed and working memory and cognitive decline in two of these constructs. Most importantly, this study has established that short-term changes in IT predict future cognitive test performance in fluid reasoning, perceptual speed and working memory, independent of baseline IT. Findings are consistent with the proposition that IT is a valid biomarker and future research should investigate the predictive validity of IT for age-related outcomes such as dependence in daily functioning, nursing home placements and mortality. (Contains 6 tables.)