[Logician]

edit forumdisplay.php, find:

PHP Code:

$getperms=getpermissions($forumid,-1,-1,$foruminfo['parentlist']);
if (!$getperms[canview]) {
  show_nopermission();
} 


After that add:

PHP Code:

$usersbirthyear=trim(substr($bbuserinfo[birthday],0,4));
if ((int)$usersbirthyear<1900 OR !$usersbirthyear)
{ eval("standarderror(\"".gettemplate("fill_birthyear")."\");");exit;}
if ((int)$usersbirthyear>1990)
{ eval("standarderror(\"".gettemplate("you_are_young")."\");");exit; 


Create a templated named "fill_birthyear" which will be displayed if he didnt fill his birtyear field.

Create a templated named "you_are_young" which will be displayed if he was born after 1990. (Edit "1990" according to your wish)

Enjoy!