The right answer is this is an example of figurative language. Twain is here using figurative language - one that uses figures of speech - in order to be more persuasive, catch the reader's attention and interest, and, most importantly, provide him or her with an enjoyable experience. Specifically, I think he is using hyperbole, on the one hand, in order to emphasize, exaggerating it, his ability to remember, and paradox, or even sarcasm, on the other hand, since, how is it possible to remember something that <em>has not</em> happened? Twain is here trying to praise youth and the abilities that it certainly entails, but also to comment on the arrogance that it brings along. Mark Twain used hyperbole and other literary devices extensively in his novels and short stories, and, in doing so, he influenced several American authors of subsequent generations. One of his famous quotations reads: “I have seen this river so wide it had only one bank.”
