1. At the point that the reproduced sound enters your ears.
2. This is the Sound Science sub-forum. If you are going to make factual assertions, they either need to agree with the science or contradict/disagree with it but ONLY if you have reliable evidence to support your assertion. Unfortunately, your assertion both contradicts the demonstrated science and you have presented no reliable supporting evidence! The demonstrated science shows that at normal/reasonable listening levels no one, good ears or not, can "pick out 24 over 16 bit depth" with commercial (competently made) music audio products. Unless, you can provide some reliable evidence to the contrary, the ONLY logical conclusion is that you arrived at this assertion simply because it has been suggested to you (in false marketing for example) or you have performed some seriously flawed test yourself. Furthermore, this conclusion has been demonstrated countless times: Audiophiles often claim they can hear the difference but without exception, on those occasions when they "put their money where their mouth is" and are reliably tested, they can't. Most commonly, it turns out they were actually comparing different masters and falsely attributing the audible difference to 24 vs 16bit.
3. This statement is also false unfortunately. The math was conjecture over 90 years ago but over 70 years ago was proven (by Claude Shannon) and therefore was no longer a conjecture. Furthermore, the entire digital age depends on that proof and it it were wrong, the digital age would not exist.
4. This too is a common falsehood peddled by some/many audiophiles. It is easily disproven by the fact that the very best systems (top commercial studio systems) and highly trained, experienced engineers can't hear the difference (given the conditions above). How then is it possible that untrained amateurs, using audiophile systems that are subpar (compared to top pro studios) are hearing a difference?
5. It would be futile if both: A. The math wasn't proven and demonstrated in practice in every digital device on the planet and B. Someone chooses to believe the marketing hype and audiophile myth and dismiss the demonstrated science. This isn't the "marketing hype and audiophile myth" sub-forum though, it's the sound science sub-forum and therefore it isn't futile, except of course to those who come here by error, thinking it is the "marketing hype and audiophile myth" forum!!
Don't you think you should find out what you're hearing BEFORE making the assertion (in a sound Science forum) that what you're hearing is a difference between 24bit and 16bit AND (falsely) "bashing" others' equipment and/or listening skills?
G