This property is defined in the mix:created node and should automatically be created when you create a page either manually or programmatically. Be sure to use the PageManager to create the page in your code.
We're using the pageManager object to create the page. After the page is created the jcr:createdBy property is taken from the user ID available in the session object which is taken from the slingRequest object.
Now is there any possibility to update/override this property programatically?