I think the problem is that you added a new field to the users table for the user's age. You made it so that there is the right number of fields, but they're not in the right order. :)
Find:
PHP Code:
mysql_query("INSERT INTO ".$prefix."users VALUES ('', '$username', '$pass1','$email','3','1', '$date', '0','','','','','','')");
Replace With:
PHP Code:
mysql_query("INSERT INTO ".$prefix."users VALUES ('', "", '$username', '$pass1','$email','3','1', '$date', '0','','','','','')");
Try that and see if new users can register. Since you put the age field before the username field and your age table only allows 3 characters, all usernames were being cut to 3 characters, thus they could never log in. The fields also most certainly did not match up.
Try that and see if it works better. :)