A. Irony
Irony expresses an incongruity or inconsistency between the actual result of a sequence of events (or one), and the expected or traditional result (and could not make sense), which it's exactly what Oscar Wilde conveys in the phrase. According to most cultures, in general, to be old means to be wise, and to be young means to be reckless and not wise enough, not experienced enough to "know everything"; but Wilde mixes those concepts up. "I'm not young enough.." is the sequence of events or set of words, and "to know everything" is the unexpected result, which at the same time makes no sense according to the traditional view.