If it’s worth saying, but not worth its own post, here's a place to put it.
If you are new to LessWrong, here's the place to introduce yourself. Personal stories, anecdotes, or just general comments on how you found us and what you hope to get from the site and community are invited. This is also the place to discuss feature requests and other ideas you have for the site, if you don't want to write a full top-level post.
If you're new to the community, you can start reading the Highlights from the Sequences, a collection of posts about the core ideas of LessWrong.
If you want to explore the community more, I recommend reading the Library, checking recent Curated posts, seeing if there are any meetups in your area, and checking out the Getting Started section of the LessWrong FAQ. If you want to orient to the content on the site, you can also check out the Concepts section.
The Open Thread tag is here. The Open Thread sequence is here.
It sounds like you're on the "Enriched" tab, i.e. a feed of posts that includes some algorithmically suggested posts. Probably you were switched over to it in an AB test. I'm not 100% sure how you were supposed to find that out. From reading that post, my guess is that via the stickied "LW Frontpage Experiments" at the top of your feed.
So, the reason you're seeing some old posts is because our recommendations engine thinks you might like them (though I understand the team working on it has put a fair bit of effort towards biasing it towards some of our good, older content, like the Sequences — which seems like a good idea to me, too).

You can switch back to the normal view by clicking "Latest" at the top of the list of posts: