The following formula could be more accurate

(DATEDIFF(yy, @dob, getdate()) - CASE

WHEN DATEPART(m, @dob) < DATEPART(m, getdate()) THEN 0

WHEN DATEPART(m, @dob) = DATEPART(m, getdate()) AND DATEPART(d, @dob) <= DATEPART(d, getdate())...