The term itself is an oblique reference to Olaf Stapledon's classic science fiction epic Last and First Men. In this book the stages of mankind are enumerated. Thus early '50s historians of fandom began to label successive generations of fans as First Fandom, Second Fandom, Third Fandom, and so forth... all the way to Seventh Fandom and beyond.
Then in 1958 a number of fans at Midwestcon realized (with some liquid inspiration) that they all had been active in fandom for more than 20 years. The rest is history. First to assume the chairmanship was Robert A. Madle.
Currently the organization allows several classes of membership. For example, a Dinosaur is a member who was active before the first Worldcon (World Science Fiction Convention) held on July 4, 1939, while Associate Membership requires provable activity in fandom for more than three decades.
There is an analogous informal society in Finnish fandom called the Dinosaur Club; the cutoff being the first major Finnish con Kingcon.
External link