[tbworld]

The problem you are having is the user condition $userinfo['showbirthday'], which alters when $age is valid in the vbulletin code. In reality all you need to check is if $userinfo['birthday'] is valid.

Using @Scanu suggestion, this is all you need -- although, that code needs to be fully vetted. I believe $userinfo['birthday'] should be properly validated from vBulletin. I would re-validate it if you run many modifications, that might tamper with it.

   // For testing purposes use an expanded format,
   //   you can always combine terms after you have it working.


    // Exempt: smods, mods, admins    
    if (!is_member_of($vbulletin->userinfo, 5, 6, 7))
    {
        // Should already be validated if comming from vbulletin.
        //   or add additional validation to be on the safe side.
        if (!empty($userinfo['birthday']) && ($userinfo['birthday'] != ' ')) 
        {  
            $birthdate = $userinfo['birthday'];
            $birthdate = explode("-", $birthdate);
              
            //Calculate Age (@Scanu's suggestion)
            $age = (date("md", date("U", mktime(0, 0, 0, $birthdate[0], $birthdate[1], $birthdate[2]))) > date("md")  
                  ? ((date("Y") - $birthdate[2]) - 1) : (date("Y") - $birthdate[2]));
           
             // $Age Check
             if (isset($age) && ($age < 18)) 
             {
                    print_no_permission();
              }    
          }      
      }