The rates of almost all causes of disease, injury, and death vary by age. Age adjustment is a technique for “removing” the effects of age from crude rates so as to allow meaningful comparisons across populations with different underlying age structures. For example, comparing the crude rate of heart disease in Florida with that of California is misleading, because the relatively older population in Florida leads to a higher crude death rate, even if the age-specific rates of heart disease in Florida and California were the same. For such a comparison, age-adjusted rates are preferable.
Age-adjusted rates are calculated by applying the age-specific rates of various populations to a single standard population. In CDC WONDER, if you choose to age-adjust rates, you must specify your standard population (or accept the default). It is good practice to specify a standard is that generally similar to the populations being compared. For example, if requesting breast cancer mortality rates for white females in Massachusetts for the period 1979-1991, you might specify “1985 Massachusetts white females” as your standard population.