I think it would be nice to include the original posting date when a related article is suggested. I'd be more inclined to check it out if I knew the article was less than a few months old. More than a year, not so likely.
Related articles attempt to be non-time sensitive, such as code snippets, solved threads, and that type of thing. Keep in mind that the solved thread associated with a particular page rarely (if ever) changes, so even if it is relevant today, it won't be so next year, and most threads gain most of their traffic over months and years.