Sites from the mid 1990's that are rarely updated but seen as a resource at believed to have "grandfathered" the organic results. As of now, most of these pages are still valid, although not frequently update. The concern going forward, as mentioned in the WebmasterWorld thread named The Google age-factor is that these sites (in 10 or so years) will be inaccurate and have dozens of broken links. The member points out "there are many many "forgotten" pages ranking high in Google and they're holding back new sites and pages that aren't in the old loop of links, nor have the benefit of the educational site PageRank."
So what shall a search engine do? With PageRank and link popularity being so important, how do these pages that are so well established sift to the bottom of the results? One member responds, "Surely Google is thinking ahead and not simply reacting to complaints?" His reasoning, "I often get emails from a site that monitors millions of sites for broken links, so it can't be too hard for Google to do likewise (if they don't already)."
As new sites pop up and link equity for those pages increase, why can't those replace the old, outdated pages over time? Especially with the growth of the Internet and Web pages these days - I believe it would be achievable. In addition, as long as the pages linking to other pages are up and current, this problem should not continue. One thing is for certain, Google and the other search engine PhDs are thinking about this.