The message is about the user last post before the post it’s attached to. It means that before the post on the picture, the last post was five years ago, and it’s meant so other users can react properly to either a new user or an old timer coming back.
Thanks for the explanation, understood. In addition to rephrasing as suggested by @RGJ, shouldn’t the time span be respective to the time of writing? When the post was written, the previous post was 2 years old. That it is five years now isn’t really the message here, is it? (If such a message makes any sense at all after this time.)