I'm not sure exactly. While doing some house-cleaning a few years ago, I found a TWA Aviator card long after TWA had already merged into AA. I was flying cheap by flying overnight courier flights cross-country in the late 80s, so it may have been from then? And then I applied for Norhtwest WordlPerks in the early 1990s, but didn't fly Northwest again for about a decade after that (and only realized by accident at that point that I already had an account with them from a decade earlier).

So there may or may not have been other programs I signed up while flying one flight and then forgot about.

The first airline program I applied for and actually kept actively using was Southwest Rapid Rewards circa 95 or 96 when a company I was working for shuttled me several times for a work project from SoCal to Silicon Valley. That was back when a certain number of paid short-hop round trips on Southwest got you an award on Southwest for anywhere in the country they flew, with last-minute seats typically available!