I was surprised when I got one; it just appeared in my inbox one day. When my account turned 11 I decided to see if /r/11yearclub existed and it did, but I had to message the mods and join manually.
I had to request to join Century Club manually too; I didn't even notice I could do it until I had 200k karma...
I joined the Century Club and it was just kind of boring. I rarely ever saw a thread I felt compelled to comment on. I suspect it's because there really wasn't much of a common interest behind it. Didn't know about the 10-year club but I suspect it would be similar, perhaps with more of a "aren't kids these days awful" vibe.
Yeah, it wasn't a bad place. I'm sure a lot of people would expect it to be full of threads about how century club members were better than the low-karma plebs or discussing tips on how to farm more karma or something, and it was nothing like that. I mainly recall it being a place where people would share tragedies in their lives and commiserate on them, knowing that they had a relatively "private" place to be discussing that sort of thing with people they considered "peers." I just didn't find it to be compelling discussion most of the time.