We weren’t middle-aged when we joined the boards.
The Dope must be making us old!
StG
(bolding mine)
Does not compute. 
No, it hasn’t.
Yes it has.
Pull the other one.
Oh, I’ve had enough of this.
There are many popular message boards on the internet mostly used by a younger demo. The SDMB is not a living time capsule of internet communication. Although most of the ones I visit are more focused (basketball, video games, cartoons, movies) whereas the Dope is more general.
I find the Dope has a good level of frequent activity, but not so much that every opinion has been “taken” by somebody else before I see a thread.
I also like the conversation style here. Again, a good mixture of serious factuality and silly snark. I find most other places that focus on science to be ridiculously dry, outlawing jokes of any kind. Then there’s the other extreme of, say YouTube comments or Facebook.
If I had to pick one reason why I am attracted to conversations on the Dope it’s the generality of it’s mission.
Because the board has conversations on many subjects, and it draws people from diverse backgrounds, I learn about topics I never would have thought to read about discussed by people who bring ideas and viewpoints alien to my own.
In this act of discovery I find joy.
I don’t quite understand your middle sentence; is that “not” unintended? Because a word-centric, long-form forum is very much something from the past, a living time capsule of how online discourse can go when it’s not restricted by length, topic or slant, persists indefinitely, and has strong moderation keeping the destructive excesses at bay.
I don’t know of any other general-interest board that isn’t blog-comment style, with no persistence and no support for long posts, essentially unmoderated unless you piss off the board owner and get banned, and isn’t heavily tilted one political way or another.
Side observation: even the intelligent younger generation (under 25) seems seduced by the idea that a hundred one-liners add up to intellectual discourse.
I frequent a woodworking site that is just about as active as the SDMB, but again it is full of old people.
But you really can’t get answers to questions about what the universe was like 1 micro second after the big bang at that message board.
For my old person story, I was talking with someone about the pre-internet days and he mentioned Napster. I said before Napster, we had to steal music the old fashioned way. Send in a Columbia House card with a fake name.