MSSQL Calcluate age from Date of Birth

This excerpt will convert a date of birth into an age. It returns NULL when no DoB is present.

CASE WHEN T.dwoDOB IS NULL THEN NULL 
 ELSE 
 CASE WHEN DATEADD(YEAR, DATEDIFF (YEAR, T.dwoDOB, CURRENT_TIMESTAMP), T.dwoDOB) > CURRENT_TIMESTAMP 
 THEN DATEDIFF(YEAR, T.dwoDOB, CURRENT_TIMESTAMP) - 1 
 ELSE DATEDIFF(YEAR, T.dwoDOB, CURRENT_TIMESTAMP)
 END 
END

Example

SELECT 
T.swmFirstName, T.swmLastName,
ircAge = 
   CASE
      WHEN 
         T.dwoDOB IS NULL 
      THEN
         NULL 
      ELSE
         CASE 
            WHEN
               DATEADD(YEAR, DATEDIFF (YEAR, T.dwoDOB, CURRENT_TIMESTAMP), T.dwoDOB) > CURRENT_TIMESTAMP 
            THEN
               DATEDIFF(YEAR, T.dwoDOB, CURRENT_TIMESTAMP) - 1 
            ELSE 
               DATEDIFF(YEAR, T.dwoDOB, CURRENT_TIMESTAMP) 
         END
   END
FROM T as tblContacts