Two examples of working with dob in MySQL as age rather than date.

mysql_dob_decider_examples.sql

SELECT dob FROM from table
-- all dob between 20 and 25 years of age
WHERE FLOOR(DATEDIFF(now(), CAST(dob AS DATE))/365.25 ) BETWEEN  20.0 AND 25.0
-- all dob equal or older than 40
AND Year(Curdate()) - Year(dob) - ( Right(Curdate(),5) < Right(u.dob,5)) <= 40
-- dob within the last six months
AND dob >= DATE_SUB(NOW(), INTERVAL 6 MONTH)