This example calculates the age. It does this by first determining the number of years between the current date, and the Birthday. Then if the birthday has not happened this year then 1 is subtracted from the number of years between the current date and the Birthday.
-- Calculating Age
-- Written by: Gregory A. Larsen Date: 2/23/2008
DECLARE @BirthDay DATETIME
SET @BirthDay = '1/22/1967'
YEAR(GETDATE()) - YEAR(@BirthDay) -
-- IF Birthday for this year has not come yet, then subtract one
WHEN (MONTH(@BirthDay) > MONTH(GETDATE()))
OR (MONTH(@BirthDay) = MONTH(GETDATE())
AND DAY(@BirthDay) > DAY(GETDATE()))
ELSE 0 END)