My dad discovered it while he was up late at night when my sister was a baby, so around late 1990, early 1991. He then told me about it, and I've been watching ever since.
I think MST3K is really just a brilliant show for so many reasons. It's really funny for almost anyone. Little kids think it's cool because the robots are cute, the villains are crazy, and they're talking in the movie theater like all the grown-ups say not to. Then as you get older you actually start to, you know, GET the references. The older you get, the more you learn, the more you listen to Frank Zappa, the closer you are to the Midwest, the funnier the show gets. You can catch something new with every viewing. Now yes, the later episodes did resort to cheap humor too often, but they were burned out, and missing a lot of the original contributing writers like Trace (Dr. F/Crow), Frank, Joel, and others. But still, MST3K is one of the best-written shows of the 1990s, and hopefully, its audience still keeps growing well after its eleven years.
EDIT: Watch out for snakes!