What I mean is the longer "forums" as a thing have been around, not the age of an individual forum. In the early days of forums, exchanges lasted a bit, but fizzled out. Each subsequent attempt since then has generally fizzled out faster than the previous. Even with automated software to track and remind, they don't last.
Why put any resources into something that's going to give and receive thin low quality content, by people who aren't experts in the forum's niche? Google doesn't index thin content like it used to.
I have found time is better spent focusing on a quality site such as good, in-depth content and promoting the site. This keeps users coming back, whereas building a site on exchanging posts... once the exchange stops (and it will) you're back to a wasteland.