Sure, it just requires a slightly more complicated SQL statement that converts the date or datetime field to a Unix timestamp. I would probably do something like this:
PHP Code:
$age = 20 // change this variable to get different ages
$age_sql = "SELECT * FROM tblTableName WHERE UNIX_TIMESTAMP(fldBirthDate) >= UNIX_TIMESTAMP(NOW())-" . $age * 31556926 . " AND UNIX_TIMESTAMP(fldBirthDate) < UNIX_TIMESTAMP(NOW())-" . ($age + 1) * 31556926 . ";";
...where tblTableName is the table and fldBirthDate is the field containing your birthdates.
A Unix timestamp is the number of seconds since Jan. 1, 1970. 31556926 is the number of seconds in 1 year (365.24 days).
I haven't tested this explicitly, so no guarantees, but I think it gets the general idea across. And I'm sure there may be a more eloquent way to do this, so if anybody knows one, please share!