Whatever the nature of aging and age-related change, it is axiomatic in gerontology that most general physiological and biological functions in older persons tend to have greater variation than in younger persons, related to greater variation in age-related change, the selective occurrence in subpopulations of specific medical conditions, varied availability of and access to optimal treatments and rehabilitation for these conditions, varied individual capacity to cope with and adapt to these changes and conditions and the nature, compatibility, and adaptability of the social and work environments. It is this variation that provides the basis for the observation that function and performance often do not correlate very well with chronological age (Masuo et al., 1998). Much of this variation may be modified by the social environment outside of work.

Another general characteristic of older populations, including older workers, is the presence of comorbidity (Gijsen et al., 2001; Brody and Grant, 2001). This refers to the distribution and co-occurrence of medical conditions that may affect health status, health risk, and adaptability to the work environment. While some active conditions may limit or preclude employment, many prevalent conditions in older workers are well controlled and do not have a substantial functional impact on worker performance. Comorbidity may, however, affect the use and timing of medical care utilization or encompass treatments that may alter workplace activities, such as somnolent or other psychoactive drugs. However, there has not been very much research on the effects of various workplace exposures on health in the presence of even controlled clinical conditions or their treatments. Also, further evaluation is needed to determine the role of comorbidity as an indicator of increased risk to various workplace exposures, requiring administrative action such as altered job placement.

In addition to acquired comorbidity and the heterogeneity in age-related change is the increasing number of children and young adults with substantial disabilities due to congenital, inherited, or acquired conditions that manifest themselves in childhood or early adulthood. Due to improved medical and rehabilitative care, as well as specialized social support, many more of these individuals are now surviving into late adulthood; some may at certain points be eligible for certain types of productive employment under certain circumstances. How their trajectories of age-related illness or disorders and dysfunction compared to others in the population has not been well characterized and should be one focus of future research in order to determine the suitability for various workplace environments.

However, as noted above, it may be difficult to predict the health and functional status of subsequent cohorts of older populations and older workers. For example, there is emerging evidence that in recent decades each succeeding cohort of older populations, from whom older workers are drawn, have overall better levels of physical and cognitive function than the



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