Measurement usually attempt to estimate the impact of functional limitations on care providers, but they often ignore differences among IADL. As noted, these measures are disability-level measures.
The age of onset of an impairment and the duration of impairment are recognized as important aspects of functional limitations knowledge and research. Aging must be considered in evaluating the functional status of a person with a disabling condition over time and in evaluating the appropriate interventions. Aging is a conception-to-death series of events that includes attaining, maintaining, and losing skills. Therefore, functional capacity changes with age. Growth and development affect the functional outcomes of interventions for infants and children with disabling conditions. No validated methods of discriminating between development and interventions in children with developmental disabling conditions exist.
The process of aging discriminates against no person. Everyone is a participant in the process of growing older—including people with disabilities. ''Nondisabled Americans are getting older; they're living longer, there are vastly more of them, and they're getting old nonfatally. In short, they're becoming more disabled.... All at once, it seems, there are a lot of formerly nondisabled people around." (Corbet, 1990) In the last two decades increasing attention has been directed toward disability and aging.
In people with disabling conditions, depending on the compensatory strategies used, secondary conditions and comorbidities, can affect functional status throughout a person's life. Secondary conditions are impairments, functional limitations, disabilities, diseases, injuries or other conditions that occur during the life of a person with a disability, where the primary disabling condition is a risk factor for that secondary condition, or may alter the management of health and medical conditions. This of course is based on the new paradigm that people with disabilities are healthy, that is a disabling condition does not imply illness and disease. Each factor in the interaction of disability and aging has the capability to become a "negative feedback loop" (Guralnick, 1994) which may lead to further disability or a new medical condition.
In recent years, a body of literature regarding the effects of aging and secondary conditions has been developing. Spinal cord injury and aging is the best developed, with information available in the areas of quality of life (Evans et al., 1994), functional changes over time (Gerhart et al., 1993; Pentland and Twomey, 1994), premature and interactive effects of disability and aging (Ohry et al., 1983; Lammertse and Yarkony, 1991; Bauman and Spungen, 1994), aging and secondary conditions (Charlifue,