How do I know? Because I link to all sorts of articles and tutorials, and over time, they disappear and the links go to pages that aren't found or are re-directed to content that is different from what I intended.
Of course, those articles aren't the same as the pictures of you doing something embarassing that you don't want your co-workers, boyfriend or mother to see. Those probably will be there forever.
One of the things I've started doing is re-visiting my old blog posts and making sure they are still good. I took this on because I know some things have changed. And some of those changes are exciting!