Since the mid-1980’s with Commodore hosted BBS’s.
I haven’t been on any boards much for the last year or so, but I’ve been Hamadryad since 1985. I had an Apple IIe, but got on Commodore and Amiga boards back in the day.
ParentSoup, 1997.
I used to love Usenet in it’s day before it was all spam and P2P, I thought it was much better than today’s message boards, because it was WILD.
Since first getting online with AOL in '94.
Me and the other programmers used swap funny punchcards for ENIAC back in the 1940’s and 1950’s.
I have been a message board person since June 2000 when I signed up here. And here is the only message board I’ve every contributed to, so I’m not really sure how much of a message board person that makes me.
I started working at Xerox’s software division in 1989, and they had a thriving internal message-board community. That was really when I started. I dabbled at Compuserve starting in the late 70s (or maybe it was the very early 80s), but it was expensive back then so I didn’t get to be on much (my parents were paying for it and it was like $10.00 an hour, so once we worked our way through the free hours that was pretty much it for me and Compuserve. 
)
Since 2000, when I got my first computer. I mostly stuck to the Everquest messageboards, though most of those are shut down or have little traffic these days.
Outside of those there’s SDMB, ATS (a conspiracy website), Mystic Wicks (pagan website). There was an atheist message board I posted at, but it shut down last year, and a hippie messageboard I can’t find anymore. 
Primarily due to evening shift boredom, I started reading message boards in 1996. I didn’t start posting until several years after that.
I joined the Harmony Central forums (musicians) in 2001, here a few years after.
ETA: Five years after, apparently.
38 years. I was extremely active on PLATO when I was at the U of I, and took the PLATO course in 1974, and stayed on until I left in 1977. I was around when PAD split into a number of topic based pads, much like today’s message boards.
I also wrote a column, about Star Trek, for the world’s first on-line newspaper, edited by The Red Sweater.
I was also on Usenet from 1985 or so, and remember when the alt.sex hierarchy consisted of one not very busy group.
Found the net through boyfriend (current husband) in 1992 via Prodigy. He used it for legit pursuits, such as buying airline tickets from “Easy Saabre.” I discovered the AOL chatrooms leading him to say “For god’s sake, It’s 2am! Come to bed!” Found the SDMB through (dial-up) AOL.
This is honestly the first messageboard that I used more than in passing. Prior to this I spent all my time online downloading programs to try out, even on a 28.8 kbps modem. Before that, I spent my time learning to use a unix shell account. I really was fascinated by Gopher.
BBSer since '87.
Technically, I logged on to a BBS a couple of years before that, but that was on a 110 baud modem. The first ANSI menu took about five minutes to come up – I decided not to make it a daily activity at the time.
I’ve been participating in one kind of online discussion forum or another since around '97 or so, if memory servers, starting with Yahoo Categories (really rudimentary Q&A forums - think Answers.com), Usenet, and others. So going on 15 years.
A little over 14 years. I started on the Animorphs message boards on AOL; I was 14 at the time. I read the original SD forums when they were on AOL, too, as well as the Nirvana and REM boards. I posted sparsely on the last two, and very sparsely on the SD forum. I posted a lot on The Haiku Year’s website, though that wasn’t really a message board.
Really, though, this has been my only board since I went to college back in 2001.
I move to Atlanta in 1994, and I know I was on several BBS’s before I moved here. So at least 17 years. Wow.
I was on some BBSs back in 95 & 96 but then nothing until I joined the Dope in 2005. The BBS were narrow interest. One was on Tolkien. But I never posted much.
Much like Voyager, I have been on programs similar to bulletin boards since about 1973.
In fact, when I was at Georgia Tech on the supercomputer Cyber 74, a bunch of us modified a BBS called ‘Forum’ that the techs at Control Data Corporation had written.
At the time, when you posted, it was your username that was tagged to your post. But, of course, you didn’t choose it. It was your billing account number, as in cmarlbdo182.
So, we created a ‘handles’ list.
Anyone here remember CB radio when you had a call sign and a handle?
We coded a CrossRef list between the utterly unpronounceable username into something others could remember.
This was, I think, 1974.
Anyway, that was when I first started posting.
Does that count?
It wasn’t the Internet, it was a mainframe that served 16 terminals (and eventually 64).
Otherwise, I would guess I would say the very earliest Usenet stuff back in the early 1980’s… maybe 81. I used lynx as a ‘browser’. Lots of gopher and some Archie.
Anyone remember ‘VERONICA’?
“Very Easy Rodent-Oriented Net-wide Index to Computer Archives”
Now I feel old. Think I’ll just drink heavily and go to sleep now.