The 25-Year Study of Disability Trends:
A 25-year study, which was published in 1996 by the National Institute on Disability and Rehabilitation Research (U.S. Dept. of Education) indicated that the numbers of disabled in the U.S. increased substantially during that 25-year period.
Here's the complete report in PDF format, if you're interested:
Trends in Disability Rates in the United States, 1970-1994.
From that report:
The proportion of the U.S. population with disabilities [severe enough to limit their activity] has risen markedly during the past quarter-century.
The data in that report showed two distinct trends:
A gradual rise, due largely to demographic shifts associated with an aging population [and] a rapid increase ... [due to] greater numbers of children and young adults reported as having disabilities.
These two demographic subgroups are the most significant, in terms of TOTAL DOLLARS SPENT, for most retail businesses.
Also, according to that report (prepared by the Disability Statistics Rehabilitation Research and Training Center, Institute for Health & Aging, School of Nursing, University of California):
The most dramatic changes in disability rates have occurred during the 1990s ... Because about 70 percent of the population is under age 45, these steep increases in disability prevalences are responsible for the recent rise in proportion of the overall population with disabilities.
The trends shown in this study have continued to increase as had been predicted. For current data on these issues, please refer to the following two resources:
National Institute on Disability and Rehabilitation Research
("NIDRR" a division of the U.S. Department of Education's Office of Special Education and Rehabilitative Services)
U.S. Disability Statistics by Year (updated through 2007, provided by the Employment and Disability Institute, a Division of Cornell University's School of Industrial and Labor Relations)
NOTE: While access to this information is free, Cornell's website requires registration and login.