Well, how do you actually determine the age of a web page? Is it the post date? How do you even find that out? Is it the last comment post date? Is it the last edit of the main post, or the last edit of a comment? How do you find this out automatically? Is it the last change the HTTP server responds with? Is it the last time the entire page has been modified? If the page is built up of multiple components like iframes, do their post dates matter? Do ads matter? If the page is dynamic, everything gets a few orders of magnitude more complicated.
Point is, it is not a trivial task at all to automatically find out the time that corresponds to the intuitive understanding of the "age" of a web page.