MikeP should get an A++ for finding that!
I went to CP but couldn't find that option. Eventually I poked around and found it under Settings/ Options. When I found this toggle, it had an explanation:
Do you want to reverse the displayed order of posts in the topic?
If checked, posts are displayed from most recent post first to oldest post last. If unchecked, the oldest post displays first, newest post displayed last.
Note: You can change this option by clicking on the name of the topic on the topic page.
The Note caught my attention. I went back to the Board Index and selected a topic at random. Right by the Title of the thread there is an arrow and a folder icon. When I clicked the title, like the Note said, the arrow changed up to down. Clicked again, and the arrow changed again down to up. I noted the dates each time. One click changed it to latest post on top (what Gunslinger didn't want) and another click switched it back (watch the arrow change) to first post on top. Then I naturally had to check to see if it applied to all posts/threads or just the one opened. When you click to change, it appears to change all.
So, you can also just click the title of any of the posts and that will also change the order for all the posts you see.