23-year-old Forrest J. Ackerman, in 1939, as a spaceman at the first World Science Fiction Convention (Nycon I). This silver-suited nobody would eventually go on to become literary agent for a number of greats, including Heinlein, Bradbury, and L. Ron Hubbard, as well as the editor of one of the late 20th century's most popular science fiction magazines (Famous Monsters of Filmland). As a prank he also wrote lesbian stories (under a female pseudonym) for the Daughters of Bilitis, America's first lesbian rights organization, which declared him an "honorary lesbian." He coined the term "sci-fi," and was part of a small and now mostly extinct group of founding fathers of geek culture who were mock-reverently (and are now actual-reverently) referred to as the First Fandom.
He did the costume as a joke. I'm pretty sure he never expected the joke to run for this long. :]
I'm a spaceman / I've got my spacesuit on / I've got my space girl / Singin' my spaceman song / I've got my moon rocket, I've got my moon rocket, got my moon rocket, got my moon rocket...