You're 60, can you not wear a suit and trainers?By this, he meant
Because you're 60 [and should know better/are too old], please don't wear a suit and trainers.It can have another meaning, though, and one that I think is much more accessible, or perhaps only available, in certain dialects:
You're 60, so is it impossible for you to wear a suit and trainers?For me, both those readings work fine for the sentence and the intonation is pretty much the same in each case. The difference is once again a matter of how 'high' the negation is in the sentence:
Is it the case thatvs
[not wear a suit and trainers]?
Is it not the case thatFor more on this, a very easy to read article (old now, but still good) is Bob Ladd's 1981 paper.
[wear a suit and trainers]?