This demonstrates that the datediff solution does not change the age at the time of the birthdate. All the age results are the same:

declare @BirthDate DATETIME

set @BirthDate = '08/27/1980'

print @BirthDate

print getdate()

SELECT 'DATEDIFF AGE = ', DATEDIFF(YEAR, @birthdate , GETDATE())

print 'TODAY IS THE TWENTY-SIXTH'

set @BirthDate = '08/26/1980'

print @BirthDate

print getdate()

SELECT 'DATEDIFF AGE = ', DATEDIFF(YEAR, @birthdate , GETDATE())

set @BirthDate = '08/25/1980'

print @BirthDate

print getdate()

SELECT 'DATEDIFF AGE = ', DATEDIFF(YEAR, @birthdate , GETDATE())

set @BirthDate = '08/24/1980'

print @BirthDate

print getdate()

SELECT 'DATEDIFF AGE = ', DATEDIFF(YEAR, @birthdate , GETDATE())

The following translates the dates into YYYYMMDD and gets the correct age, changing at at the month and year of the birtdate.

DECLARE

@CURRENT_YEAR INTEGER

, @CURRENT_MONTHS INTEGER

, @CURRENT_DAYS INTEGER

, @CURRENT_YYYYMMDD INTEGER

, @AGE_INTEGER INTEGER

SET @CURRENT_YEAR = DATEPART(YEAR, GETDATE())

SET @CURRENT_MONTHS = DATEPART(MONTH, GETDATE())

SET @CURRENT_DAYS = DATEPART(DAY, GETDATE())

SET @CURRENT_YYYYMMDD = (@CURRENT_YEAR * 10000) + (@CURRENT_MONTHS * 100) + @CURRENT_DAYS

SELECT 'CURRENT_DATE = ' , @CURRENT_YYYYMMDD

declare @BirthDate DATETIME

, @YEARS INTEGER

, @MONTHS INTEGER

, @DAYS INTEGER

, @BIRTH_YYYYMMDD INTEGER

set @BirthDate = '08/27/1980'

set @YEARS = DATEPART(YEAR, @BIRTHDATE)

SET @MONTHS = DATEPART(MM, @BIRTHDATE)

SET @DAYS = DATEPART (DD, @BIRTHDATE)

SET @BIRTH_YYYYMMDD = (@YEARS * 10000) + (@MONTHS * 100) + @DAYS

SET @AGE_INTEGER = (@CURRENT_YYYYMMDD - @BIRTH_YYYYMMDD) / 10000

SELECT 'BIRTH DATE = ' , @BIRTH_YYYYMMDD

SELECT '@AGE_INTEGER = ' , @AGE_INTEGER

PRINT 'TODAY IS THE TWENTY-SIXTH'

set @BirthDate = '08/26/1980'

set @YEARS = DATEPART(YEAR, @BIRTHDATE)

SET @MONTHS = DATEPART(MM, @BIRTHDATE)

SET @DAYS = DATEPART (DD, @BIRTHDATE)

SET @BIRTH_YYYYMMDD = (@YEARS * 10000) + (@MONTHS * 100) + @DAYS

SET @AGE_INTEGER = (@CURRENT_YYYYMMDD - @BIRTH_YYYYMMDD) / 10000

SELECT 'BIRTH DATE = ' , @BIRTH_YYYYMMDD

SELECT '@AGE_INTEGER = ' , @AGE_INTEGER

set @BirthDate = '08/25/1980'

set @YEARS = DATEPART(YEAR, @BIRTHDATE)

SET @MONTHS = DATEPART(MM, @BIRTHDATE)

SET @DAYS = DATEPART (DD, @BIRTHDATE)

SET @BIRTH_YYYYMMDD = (@YEARS * 10000) + (@MONTHS * 100) + @DAYS

SET @AGE_INTEGER = (@CURRENT_YYYYMMDD - @BIRTH_YYYYMMDD) / 10000

SELECT 'BIRTH DATE = ' , @BIRTH_YYYYMMDD

SELECT '@AGE_INTEGER = ' , @AGE_INTEGER

set @BirthDate = '08/24/1980'

set @YEARS = DATEPART(YEAR, @BIRTHDATE)

SET @MONTHS = DATEPART(MM, @BIRTHDATE)

SET @DAYS = DATEPART (DD, @BIRTHDATE)

SET @BIRTH_YYYYMMDD = (@YEARS * 10000) + (@MONTHS * 100) + @DAYS

SET @AGE_INTEGER = (@CURRENT_YYYYMMDD - @BIRTH_YYYYMMDD) / 10000

SELECT 'BIRTH DATE = ' , @BIRTH_YYYYMMDD

SELECT '@AGE_INTEGER = ' , @AGE_INTEGER