DECLARE @BIRTH_DATE DATETIME, @STPR_START_DATE DATETIME

SET @BIRTH_DATE = '1975-01-07'

SET @STPR_START_DATE = '2013-01-06'

SELECT DATEDIFF(YEAR, @BIRTH_DATE, @STPR_START_DATE)

Result: 38

SELECT DATEDIFF(YEAR, @BIRTH_DATE, @STPR_START_DATE) - CASE WHEN MONTH(@BIRTH_DATE)*100 + DAY(@BIRTH_DATE) > MONTH(@STPR_START_DATE)*100 + DAY(@STPR_START_DATE) THEN 1 ELSE 0 END AS AGE

Result: 37