mysql> SELECT  id, email, birthday, birthmonth, birthyear, str_to_date(concat(birthmonth,"/",birthday,"/",birthyear), '%m/%d/%Y') AS birthdate FROM users HAVING birthyear != 0 and birthdate > str_to_date('00/00/1990','%m/%d/%Y');
+----+-------+----------+------------+-----------+------------+
| id | email | birthday | birthmonth | birthyear | birthdate  |
+----+-------+----------+------------+-----------+------------+
| 11 | o@o.o |       14 |         12 |      1993 | 1993-12-14 |
+----+-------+----------+------------+-----------+------------+
1 row in set (0.01 sec)

mysql> SELECT  id, email, birthday, birthmonth, birthyear, str_to_date(concat(birthmonth,"/",birthday,"/",birthyear), '%m/%d/%Y') AS birthdate FROM users HAVING birthyear != 0 and birthdate < str_to_date('00/00/1990','%m/%d/%Y');
+----+---------------------+----------+------------+-----------+------------+
| id | email               | birthday | birthmonth | birthyear | birthdate  |
+----+---------------------+----------+------------+-----------+------------+
|  1 | mikkel@mikjaer.com  |       17 |          5 |      1983 | 1983-05-17 |
| 12 | a@a.a               |        3 |          3 |      1980 | 1980-03-03 |
| 16 | toby_uknow@yahoo.dk |        6 |          4 |      1988 | 1988-04-06 |
+----+---------------------+----------+------------+-----------+------------+
3 rows in set (0.00 sec)