It is hard to imagine this actually being given as a speech, even though Twain certainly writes it as if it were one.
If you are talking about the imaginary audience for this, I would say that Twain is pretending that he is addressing a crowd of college students. The reason I say that it is addressed to that level of person is that Twain uses a lot of big words that would likely be over the heads of a younger group. I suppose it could be a high school graduation, though, because not so many people went to college back when Twain was writing.
So, I guess I would say that it is a graduation speech aimed at a high school audience or maybe a "welcome to college" speech.
I don't think it is a college graduation speech because it is too late for them to learn to lie and such -- this is advice that is best given to people who are still in their formative years.