Trajectories of Functional Health and Its Associations With Information Processing Speed and Subjective Well-Being: The Role of Age Versus Time to Death

Functional health declines with advancing age, which is supposedly the consequence of both normal, "primary aging" as well as of mortality-related, "tertiary aging" processes. To better understand the independent effects of both processes, we investigated how age and time to death relate to changes in functional health over up to 12 years. Additionally, adopting the disablement process model for an end-of-life perspective, we investigated if both age and time to death moderate associations of information processing speed and subjective well-being with functional health. Data from the German Ageing Survey were used. Our sample consisted of 578 participants who had died between 2002 and 2016 (mean age at death: 76.59 years, range 45-95 years). Information processing speed was measured by the Digit Symbol Substitution Test. The well-being indicators assessed were positive affect and depressive symptoms. Based on longitudinal multilevel regression models, we found that functional health significantly decreased over time in study. Approaching death was a stronger predictor of functional health decline than was chronological age. Regarding moderation effects, controlling for gender, education, and multimorbidity, individuals closer to death at baseline revealed stronger associations of both depressive symptoms and information processing speed with baseline functional health, whereas these associations were not moderated by chronological age. Our findings suggest that change in functional health is more strongly affected by time to death than by chronological age. Moreover, there may be a growing importance of cognitive resources and well-being for functional health with advanced "tertiary aging," but not with progression of "primary aging."