I've been involved in a loose-knit campaign for 23 years now...and the campaign had been going on for several years before I joined. At that point, I was 17, and most of the other players in the group were in their 20s or early 30s. Now, I'm almost 40, and the others who've been in it from the beginning are in their 50s.
By loose-knit, we've had several DMs share the duties over the years, most players have multiple characters, and we've had a lot of players enter and exit the group over the years. But, it's all in one (homebrew) campaign world. There's currently 4 players who've been in the campaign for 20+ years; all of us have at least one PC who we've been playing in the campaign for that long.
The campaign was originally AD&D; over the years, we've converted to 2E, 3E, and now 3.5.
Once upon a time, we played nearly every Saturday (and from about 1pm until 10pm or later most times). Now, we're spread out over two states, and only play together a half-dozen times a year...but, two of those gatherings are multi-day marathon sessions.