Could the sytem perhaps notify the user if he by mistake is trying to answer a apost tahts more than a year or so old... I mean it would spare some users out of confusion.
Offcourse it does not happen all that much. BUt every 2 months or so it does.
jsut an idea 
Briliant idea that came to mind.
Or even beetter could the backgrounds of the posts turn yellow as in old papaer thats reacting with the acids in the bleaching process (note moredn nor old paper wont get yellow because of this porcess, the yellow of those is just s***), hehe bletched out texct and dirt blotches.