Theoretically you have the ability, in your User CP, to change the way you want things displayed. However, it doesn't seem to be working, or staying the way you set it, on all of the forums. I've noticed that but just thought it was something they are working on.
What I usually do is just click the little arrow just to the right of Last Post if it's not displaying the way I want it. That will instantly reverse the display order.
You can always send Len or Jim a PM and ask them about it.
It is a function of the Forum configuration. I sorted many of them out the other day, but may have missed some. Please post what forums display old messages at the top and new ones at the bottom so that Len can fix them. Most forums display the newest messages first so that you can find and reply to them easier.