age = (somedate - birth) / 365.25;

where somedate and birth are SAS date variables (or constants or expressions). Clearly this usually doesn't return an integer and therefore it is not stating an age according to colloquial usage. That problem can be addressed by:

age = floor( (somedate - birth) / 365.25); 

age = floor( (somedate - birth) / 365.2422); 

The Julian calendar, which was introduced in 46 BC, gave every fourth year 366 days. Because this slowly causes a discrepancy between the calendar and the seasons, Pope Gregory XIII proclaimed the Gregorian calendar in 1582. The new rule provided that every fourth year will have 366 days, except for years divisible by 100 but not 400. Thus the year 2000 will be a leap year, but 2100 will not.

How SAS Date Functions Can Help
As we've seen, the Gregorian calendar, and hence an integer count incremented on an anniversary date, cannot be modeled with simple arithmetic. A completely accurate approach requires coding all of the rules for which years are leap years and all the rules for the number of days in each month.

This is where SAS software's date functions help. Date functions such as intck and intnx have the needed rules built in. Particularly, intck returns intervals that correctly account for the idiosyncrasies of the Gregorian calendar. However, a little tweaking is necessary to get exactly what we need.

Because intck alone won't produce the number of years between successive anniversaries given an arbitrary birth date or starting date, a tweak is needed to find how old the person is in months. Then, simple arithmetic will turn this number into what we need -- years that always increment on the anniversary date.

Again, consistent with common usage, we want the number of months always to be an integer and we want it to increment exactly on the same day each month (or on the first day following a month that is too short for the same day to occur). Generally, the expression

intck('month',birth,somedate)

intck('month',birth,somedate) 
- (day(somedate) < day(birth))

A one-line solution
Converting months to years, we get:

age = floor
((intck('month',birth,somedate)
- (day(somedate) < day(birth))) / 12); 

%macro age(date,birth);
floor ((intck('month',&birth,&date)
- (day(&date) < day(&birth))) / 12) 
%mend age;

age = %age(somedate,birth); 

age = %age('28aug1998'd,'24mar1955'd);
put age=;  

AGE=43

1. The birthday is February 29, and during non-leap years the person celebrates the birthday on February 28. The solution described here would treat the birthday during non-leap years as March 1. In a random population this should affect at most one out of 1,461 persons, or less than 0.07 percent of the population. If desired, extra lines of code can accommodate this situation.
2. A person's age is to be calculated at a time in history when, in some particular country, the Gregorian calendar was not in use. Beginning with the earliest date that is valid in SAS software -- January 1, 1582 -- SAS software uses the Gregorian calendar. That is the year that France, Italy, Luxembourg, Portugal, and Spain replaced the Julian calendar with the Gregorian. (The Gregorian calendar was first implemented so that the day after October 4, 1582 was October 15, 1582. Nevertheless, SAS software recognizes 31 days in the month of October, 1582.) While the rest of Roman Catholic Europe switched shortly after1582, the United Kingdom and its colonies did not move to the Gregorian calendar until 1752. Many other countries switched even later, including the Soviet Union in 1918 and Greece in 1923. Some historic dates therefore might be handled in a misleading manner -- a problem which, it should be noted, is true of any use of SAS dates in such instances. Nevertheless, given likely scenarios, age will be computed correctly in every country and era.