Application of age-adjustment to a rate allows a way to make fairer comparisons between groups with different age distributions. For example, a county having a higher percentage of elderly people may have a higher rate of death or hospitalization than a county with a younger population, merely because the elderly are more likely to die or be hospitalized. The same distortion can happen when comparisons are mage between races, genders, or time periods. Age adjustment can make the different groups more comparable.
A "standard" population distribution is used to adjust death and hospitalization rates. The age-adjusted rates are rates that would have existed if the population under study had been distributed by age the same way as in the "standard" population. Therefore, they are summary measures adjusted for differences in age distributions.
In MSTAHRS results where age-adjustment is utilized, one can acquire age-adjusted rates using the U.S. Census years 1940, 1970 or 2000 standard populations. Currently, the National Center for Health Statistics recommends that the U.S. 2000 standard population be used. However, if you compare rates from different sources, it is very important that you use the same standard population on both sides of your comparison. It is not legitimate to compare adjusted rates which use different standard populations.
The National Center for Health Statistics has an excellent article
that refers to methods of age adjustment and issues regarding year 2000 standardized
population.