I think we did a bit of research on this on the 8-bit Facebook group… the Rainbow comp was flagged up there as both a very early and very notable competition (it spawned a whole series of related books). There were a few “local micro group” competitions, but I’m not sure if we found much earlier, with a wider reach… I will check.
Edit: Yes, it was June 1982 for the first Rainbow competition. The earliest UK competition we found was a PCW magazine one from May 1983; which I’m not sure I ever managed to find any results from at the time we were talking about it (as the PCW archive wasn’t complete).
There were also a whole related category of “adventure playing” competitions too; but that’s another thread, I think.
Is IFcomp the “oldest active game competition”… Well, it really depends on your definition of “game”… If you’re talking “videogame” then, probably yes. But you’re ignoring the Spiel des Jahres… apparently running since 1978. An extremely important competition in the boardgame and tabletop industry.