Look at the time of registration and then look at when their last posts were made. This is actually my original account that I made when I was younger and a lot less bright, I somehow forgot the password and no longer had access to the e-mail account with which is was connected so I decided to to start a new account.
Years after I created my current account I started trying out some passwords and seeing if I could log into my old account again just for fun, and eventually I figured it out! Since I no longer needed the old account I disabled it and it made all of the posts that were made by my old account turn into guest posts.
Strangely, someone else has somehow brought it back to life even after I disabled it a few years ago. I wonder how they did it and why they would even bother.