I posted this as an answer to Why are the sidebar stats for a question now under the title?, but that was removed as being off-topic for that post. So, I'm bringing it to its own space.
I expect that this change will result in many more "I know this post is old...." comments in answers, or users declining to even interact with older posts. The "thou shalt not necropost" convention is burned into people from getting yelled at in forums. (That is: replying to or editing anything in an "old" thread, where "old" is often as recent as six months, is deeply frowned upon on many web forum communities.)
Of course, that convention is not what we want on Stack Exchange. We have several badges which encourage improvements to older posts (both answers and edits), and we mark incoming posts as duplicates of older ones if the older post is appropriate regardless of age. (See Is it worth my time answering old posts? and Closing posts as duplicate of very old questions, among others.)
By putting the age of the post so, so prominently, the design encourages users to expect forum-like behavior towards older posts. Was this tested against this outcome?
At the very least can we review in a few months to see if this effect is as I suspect it will be? (That is, specifically look for answers mentioning the age of the post, and also look for a decline in activity around older posts.)