mysql> SELECT  id, email, str_to_date(concat(birthmonth,"/",birthday,"/",birthyear), '%m/%d/%Y') AS birthdate FROM users WHERE birthyear != 0 HAVING birthdate < str_to_date('00/00/1990','%m/%d/%Y');
+----+---------------------+------------+
| id | email               | birthdate  |
+----+---------------------+------------+
|  1 | mikkel@mikjaer.com  | 1983-05-17 |
| 12 | a@a.a               | 1980-03-03 |
| 16 | toby_uknow@yahoo.dk | 1988-04-06 |
+----+---------------------+------------+
3 rows in set (0.00 sec)