[Logician]

It is not easy to change the forum behaviour because it is not only forumlisting page that you can access a forum. It appears in searching page, user cp etc. so it would normally require a lot of hacking.

However you can apply this clever trick:
Create 2 new usergroups other than "registered" (usergroup 2).
Set permission so that first new usergroup can not access to this forum and the second new usergroup can access this forum.

Then add this line to your forum/global.php: (say at the end before ?>)

PHP Code:

if ($bbuserinfo['usergroupid']==2 AND $bbuserinfo['joindate']>strtotime("-3 months") )
{
 $DB_site->query("UPDATE user SET usergroupid=X WHERE userid=$bbuserinfo[userid]");
} 


Replace X with the new usergroupid that can access the forum.

This hack will make sure registered users usergroup will automatically change to X when they are 3 months old. If you don't want this for specific users, put them in your other new usergroup. If you want to grant immediate access for certain users, change their usergroup to X in your admin CP.

Hack is not tested, use at your own risk.

Enjoy.