column default value settings do not work for me

I have a sharepoint document library named products that have lots of documents. I create two folders in this document library for year 2014 and 2015. Now I want to add new column(year) to this document library. I create new content type and define year column and add this content type to product document library.then I go to "column default value settings" in library settings and set default value for year in both folder 2014 and 2015. But when I come back to document library, year column dosen't have any value!

How I can set default value for year column for documents that created before in product document library?