CreatedBy field is not getting updated for pages Powershell

Hi,

 

I am trying  to update createdBy field using powershell for pages in pages library and also for documents in documents library.I tried to update  createdBy field for a page in pages library .

Below is my code for updating createdBy:

 

 

 Above code is working  for  list item (custom lists) but  not for Pages and documents in Pages library and Document library