Cognitive and Physical Functions as Determinants of Delayed Age at Onset and Progression of Disability

Disability is generally seen as the outcome of multiple comorbid chronic conditions that affect people as they age and represents the cumulative impact of age-related conditions on a person’s ability to maintain essential activities of daily living (ADL (1)). This conceptualization has resulted in two basic approaches to investigating disability and its determinants. The first approach has emphasized maintenance of independence in the ability to live in the community or to perform basic self-care tasks. The second approach has focused on late-life disability as a progressive process that can be linked to the gradual increase in organ system pathologies and chronic disease processes (2). Disability is a complex process with factors that influence transitions into a disabled state and factors that influence progression toward more severe levels of disability. The theoretical models of disability suggest that age-related transitions and progressive changes occur at various stages in the degenerative aging process (3,4). On the biological aspect of disability, pathology has to accumulate to a sufficient degree to produce organ dysfunction and impairment, which is often a function of age; impairment in one or multiple organs or domains has to reach a certain level of severity in order to produce functional limitations (5,6). Moreover, changes throughout this process may also be conceptualized as age-related transitions from nondisabled to disable state followed by progressive change in the number of functional limitations. Few investigations to date have captured and systematically examined these two aspects of the disability process simultaneously (7,8). In addition, with the exception of specific demographic characteristics, such as sex and race (9), previous research has tended to consider the influence of risk factors for disability transitions or progression as age invariant, that is, the magnitude of the association between a specific risk factor and changes in disability status as constant across age (10). However, age-related degenerative transitions provide important clinical and pathological changes in the health status of older adults. Thus, in this study, we propose a two-part regression approach to model transitions to disability and progression of disability simultaneously to see if certain risk factors differentially affect transitions and progression. In addition, instead of modeling incident disability as a function of time since baseline, we model onset as age at first occurrence of disability. This allows us to more specifically characterize the influence of age on the relationship between risk factor and onset of disability. In this investigation, we focus on two well-known determinants of aging-related disability: cognitive function and physical (or motor) function. Poor cognitive function is generally seen as a strong risk factor for late-life disability (11–14), although its potential differential impact on onset or progression has not been studied previously. Similarly, physical function is another well-established predictor of changes in disability status in older age, with better physical function being associated with lower risk of disability (15,16). In addition to these two factors, gender, race, education, body mass index, and history of chronic health conditions may influence the risk of disability among older adults (17–19) and need to be adjusted for in analyses. Therefore, in this article, we will study the role of cognitive and physical functions on the age at onset and progression after onset of disability among older adults after adjusting for above-described confounders.