It works for me - “What do you live by?” is my first listed thread in ascending order. It was last in descending order.
Don’t forget to go to the last page in both searches.
It seems to me that anybody who has posted to fewer than 750 + 750 = 1500 threads is going to find an overlap in the ascending and descending versions. One “trick” I have used when I’ve wanted to see the whole range (in my case since June, 2003) of threads I’ve posted in, is to adjust the “number of replies” number up to maybe 50 so the “junk threads” don’t appear. I can normally get down to less than 30 pages that way. Just for what that’s worth.
There you go! Been busy posting, eh? 
Mine goes all the way back to 8/23/12 when I was looking for a program to wiggle my mouse. I still occasionally use that.
/On a side note, I’ll be coming up on 10 years in May here.
That’s some forward-looking posting right there. 
Pfft, I meant the year 12. Back when the Dope was on papyrus and peeled grapes were fed to Cecil the Great.
Yep. Those were the good old days.
I don’t have a hell of a lot of posts so mine goes all the way back to Feb 1, 2003 regarding the Space Shuttle Columbia disaster and the space program.
I see what you did there.
Or I see what I want to see; either way I’m amused.
Did you mean oldest *post *or oldest thread?
I’ve been here 8+ years. I have ~6100 posts to my name, averaging 2.00 per day. I’ve started 63 threads.
My current 750th most recent post (ie the naive search) was http://boards.straightdope.com/sdmb/showpost.php?p=13838981&postcount=16 from 2011.05.24
My earliest thread is Correct Pronunciation of Name "Iyengar" - Factual Questions - Straight Dope Message Board from 2003.11.24.
It appears my posting rate has been much higher in the last 12-18 months than it was previously.
The first real result addressing me is from 2003.
Alas, anytime someone wants to rant about Tonya Harding, there’s an inevitable reference to her as a trailer park queen or some such. That tends to screw up vanity searching for me a bit.
Good stuff there, LSLGuy, and it’s the 2011.05.24 one that I was after. And you’re right that I should have specified thread over post in the OP. The difficulty with locating the oldest post is that the usual ordering of the “plain vanilla” searches is by “last posting date” so that earlier posts in that thread have to be subjected to further (manual) searching to locate the earliest by a given User. You can be sure it’s your oldest if you started the thread, but otherwise you have to do some digging.
I’ve been meaning to check the Archives to be sure I’m correct that they are in “original post date” order. The problem there (unless there’s a way around it) is that the Archives are by the specific forums and finding the oldest post in a forum may not be the truly oldest for a given User. I ran into that problem when I was trying to find the first SDMB post that referred to the 9/11/2001 events. Those posts were scattered over almost every forum that day. I eventually narrowed it down but it took some work.
Bummer!
I’m not quite sure what you’re looking for, but the oldest posts shown for me are from August 1999. Given Der Tris and Ike Witt also track that far back, and I’m pretty sure each of us have posted in way more than 750 threads, I guess that’s about the most the search function can do. I’d already been a member for several months before that.
It’s still not clear to me what you’re asking for or why.
The naive search you specify returns the 750th oldest post a person has made. Some OCD insomniac could knock out 750 posts in a single day, despite having been a member since '99. And his/her naive search would return “yesterday” as the date of the 750th oldest post. So the date returned by the naive search is really a measure of how heavily somebody has recently been posting, not a measure of how long they’ve been a member or guest.
If you’re really looking for “Who has been an occasional poster with the slowest posting rate coupled to the longest membership?”, then the person with the oldest date on the naive search is your winner.
By switching the sort order to “oldest first”, you retrieve the oldest 750 posts, starting with the user’s first post. The 750th one on that list is really a measure of how prolific they were when they first joined. If they were the OCD insomniac it could be the day they joined (net of Mod action to squash a mentally ill newbie).
For anybody with over 1500 posts, there’s that group of posts in the middle which is hard to retrieve via search. The “years ago” option lets you wade into them, but with lots of duplication & overlap from one result set to the next.
And yes, you’re right that search actually finds & sorts threads you participated in by the last-posted-to-by-anyone date which isn’t really the same thing as finding & sorting *your *posts by your posts’ individual posting date. I ignored that distinction in what I wrote above. But most threads don’t live more than a few days & assuming we’re talking about dates years or more in the past, the difference is IMO negligible.
The reason I was interested in the oldest thread from the “naive search” was just to compare posting activity among those who cared to share that information. I have several threads asking similar personal details like day of the week you were born, polls on quite a few relatively trivial details and that sort of thing. Just curious about that sort of thing.
Thanks for your views on the ways to go after old posts and threads.
Thanks for the reply. I hope I didn’t come across as jerkish. I’m a big fan of random curiosity questions too. I can spend hours following links on wiki. Not today though 
In one of my careers I field a lot of questions from customers who ask bad questions, not realizing that the question they ask & the answer they’ll get won’t really improve their understanding of the problem they really have. Your OP & subsequent comments pricked up my radar a bit. Hence my comments.
Thanks to you, too, for the reply. It’s not just this thread where I have had trouble making a simple request that winds up sounding complicated. I have yet to be “pitted” but I did get some strongly worded critiques of my posting style and I have tried to adapt to those concerns. Obviously, I’m still being fuzzier than I mean to be.
My main concern in the previous response was that it sounded as if you thought I was meddling or spying or trying to get people to reveal private data or something else nefarious. Glad you relieved that concern to some degree. That’s far from the truth of the matter. I am curious about other Dopers and their preferences and tastes and interests, but I don’t ever mean to pry. If somebody responds to a “public poll” I have started, I assume they are aware of what they are revealing. Same with topics such as this one.
It’s probably obvious but if I really wanted or needed to know these details about anyone in particular I would just do the search(es) without saying anything about it. At least this way, anybody who responds ought to know what they’re sharing, and those who have chosen not to reply are “safe” from my prying eyes!
Thanks again, LSLGuy, for your comments.
My oldest two are:
My ass hurts
Followed up by my oldest:
SDMB Crushes! Ge em while their hot!
I’m unable to determine what to make of this…