[Original Article: A script to sort the Downloads folder by download date • comment by: nateb]
And then save the script in ‘script’ format in the directory ~/Library/Scripts/Folder Action Scripts (you may need to make this directory if it doesn’t already exist).

(Per http://apple.stackexchange.com/questions/58145/how-to-add-folder-actions-in-mountain-lion …

Source: Mac OSX Hints