menintights wrote:
Some prefer the term "rhetorical question," and its purpose is exactly as you've described. It's even explicitly stated on Wikipedia.
This is the comman usage of a rhetorical question:
Wikipedia wrote:
Rhetorical questions encourage the listener to think about what the (often obvious) answer to the question must be. When a speaker states, "How much longer must our people endure this injustice?", no formal answer is expected. Rather, it is a device used by the speaker to assert or deny something.
And this is how your question plays out (my bolding):
Wikipedia wrote:
Such questions are used rhetorically, so that the question limits direct replies to be those that serve the questioner's agenda.[2] The traditional example is the question "Have you stopped beating your wife?" Whether the respondent answers yes or no, he will admit to having a wife, and having beaten her at some time in the past. Thus, these facts are presupposed by the question, and in this case an entrapment, because it narrows the respondent to a single answer, and the fallacy of many questions has been committed.
Your question demands an answer and forces him to "admit" he is 18 even if he is actually 21.