The first post ever, as well as a significant number of early posts, are long gone; there was a forum beta test period which was mostly wiped clean when the forums went public.
I believe the oldest posts/threads still currently viewable on the forums are a number of Rule Pages and Official Posts, mostly currently authored by Loki, though many were originally written by Jeckel. There are a fairly large number of Staff and Forum Tester threads still around from before it went public, for instance, a lot of Charmander's The Island RP posts. After that, the earliest posts and threads made by non-staff/testers were on September 26th.
Oldest Visible Post on the Forums
The oldest Official Staff post I can find is
The Random Images and Videos Thread, Thread #46 and Post #216 made by Loki on May 9th.
Oldest Non-Staff/Tester Post on the Forums
The earliest non-staff
post I can find is
this, made in the Official Welcome thread by Mystery at post count #2319.
Oldest Non-Staff/Tester Thread on the Forums
The earliest non-staff
thread I can find is
this, The Stormfury Suggestion thread by Drain is lief, thread #353 and post #2321; it has the distinction of being both the first thread I can find AND the oldest Suggestion.
Oldest Suggestion on the Forums
As said above, the
Stormfury Suggestion by Drain is lief
Oldest Art Thread on the Forums
Infinity Mage's pixel art
thread, thread #364 and post #2362
Oldest Mod/Tool on the Forums
A
Terraria Inventory Editor, posted by Shoot, thread #368 and post #2373
Oldest Guide on the Forums
My
100% Terraria Completion Checklist; I posted it on the forums more than a month before release to see if the massive amount of text and formatting would break anything, it comes in at thread #171 and post #993.
The oldest non-staff Guide is actually NOT found in the Guide section, but in Art; its Balduran's
Beginner's Spriting Guide, thread #372 and post #2380.
Oldest Forum Game on the Forums
Ban the User Above You, started by Zrin, at thread #355 and post #2326.
Oldest Visible SG on the Forums
Marcus101RR's
L6D Server SG, SG #7, which has honor of being the oldest still active SG by a
significant margin. The next closest is the Adult Swim SG, #18, and that is more or less official, meaning in this case, a non-staff SG pre-empts even the staff SG. There is also the never-active White Knight's Balance Discussion SG, #17.
The extremely rapid rate at which some of these Suggestions, Art Threads, and other such items were posted shows how utterly prepared some users were . . . it appears the very first thing they did was race to a specific section and post their relevant thread. The Stormfury suggestion was literally the 3rd public post on the forums I can find; that's motivation.