Good grief! You don't know much about sex either! Men in their fifties do not grow impotent unless they are ill or on certain kinds of medication (which were not available in Austen's time anyway). Anyway, Knightley tells Emma "I was sixteen when you were born" so 29 when she was 13. When Emma turns 30, Knightley will be 46. Nowhere near decrepitude. As to your last questions, there's no way that Emma would be unfaithful to Mr Knighley. Where would she find the privacy or the 'stud' to carry on adultery when she is mistress of Donwell Abbey? Knightley would not look the other way if that happened by some remote chance; he was already in the habit of correcting her when necessary.
I suggest you find something else about which to speculate.
