Converting a birth date to an age.
IF [Contactbirthday]>DATEADD('year',-DATEDIFF('year',[Contactbirthday],TODAY()),TODAY()) THEN
DATEDIFF('year',[Contactbirthday],TODAY())-1
ELSEIF YEAR([Contactbirthday]) <= 1900 THEN NULL
ELSE DATEDIFF('year',[Contactbirthday],TODAY())
END