Given two dates, how do I determine an age?

There are a few considerations here. Most people funnel into big mathematical equations, dividing the datediff in days by 365.333333333 and doing all kinds of logic to equate that to an age. In addition, we need to be wary of leap years and treat those cases differently. While a leap year baby's true age might technically be in the single digits, the more important piece of data (usually) is that x number of years have passed since they were born. Here is an example in VBScript: 
 

<%      ' use DateSerial(y,m,d) to avoid locale issues      date1 = DateSerial(1974,2,24)      date2 = DateSerial(year(date), month(date), day(date))        ' make sure we have a valid date!      if date2 >= date1 then            ' determine if leapYearBaby          if month(date1) = 2 and day(date1) = 29 then              leapBaby = true          end if                 ' get absolute number of years          ageInYears = cint(datediff("YYYY", date1, date2))            ' get date1's month and day in terms of date2's year          date1alt = dateadd("yyyy", ageInYears, date1)              if date1alt > date2 then              ' their birthday hasn't hit yet in date2's year              ageInYears = ageInYears - 1          end if            if leapBaby = true then              ' need to format output slightly              yearsPassed = ageInYears              ageInYears = ageInYears \ 4          end if            response.write "Age: " & ageInYears          if leapBaby then response.write " (" & yearsPassed & " years since birth)"      else          response.write "Invalid date."      end if  %>

 
And here is an example in T-SQL, which also accounts for leap year birthdays (and thanks to Steve Kass for correcting my earlier script): 
 

DECLARE @birthdate SMALLDATETIME, @endDate SMALLDATETIME  SET @birthdate = '19740224'  SET @endDate = GETDATE()    SELECT DATEDIFF  (      YEAR,      @birthdate,      @endDate  ) - CASE      WHEN 100 * MONTH(@endDate) + DAY(@endDate)      < 100 * MONTH(@birthdate) + DAY(@birthdate)      THEN 1 ELSE 0 END

 
(You'll probably want to put this into a function, rather than calculating it inline, if you're using SQL Server 2000.) 
 
Finally, here is a solution in Access, given a table named DOBs with a column named dob (with only date, and no time information -- or time set to midnight): 
 

SELECT      dob,       DateDiff      (          "yyyy",          CDate(dob),          Date()      )      - IIF      (          DateAdd          (              "d",              Day(dob)-1,              DateAdd              (                  "m",                  Month(dob)-1,                  CDate(year(Date()) & "-01-01")              )          )          > date(),          1,          0      ) AS age  FROM      dobs