I thought all threads show oldest first (post #1) with the most recent post at the end, possibly on another page.
Or do you mean the list of threads? "Sticky" notes are pinned to the top of a forum, even if older. These are threads that the moderator has decided are important info for quick/easy access. Otherwise, I believe the default is based on the most recent post in a thread -- so a thread may be 2 years old but someone replied to it an hour ago and it's listed near the top.
There are 2 ways to adjust the sort order:
By clicking on the light blue bar at the top of the list. Sort options include title, start date, number of replies, number of views, or date/time of last post. I have sometimes inadvertently clicked on the bar and caused a resort. You can click on the column heading to re-set.
Or another way to adjust thread sort order is at the bottom of the list. In light gray, below where it says "Showing threads 1-30 of 23,466." In the center is "Thread Display Options" and you can adjust the sort with that.