I [am] this day fourscore years old
Not that that day was precisely his birthday, but that he was about such an age, very little under or over; very probably he was full that age:
[and] can I discern between good and evil?
signifying, either that his intellectual powers were impaired, and could not distinguish what was right and wrong, and so could be of no service to David in his privy council, or any court of judicature, he was fit for no post or office under him; or that his natural senses were not quick to distinguish between good and bad, particularly between good and bad food, which he could neither discern by smell nor taste, as follows:
can thy servant taste what I eat or what I drink?
have any gust or relish of, or take any delight or pleasure in eating and drinking; so that the most delicious food, and richest wines, and all the dainties of a king's table, would signify nothing to him:
can I hear any more the voice of singing men and singing women?
either being so deaf that he could not hear them at all, or however not with any delight; the evil days being come upon him, in which he could take no pleasure in the diversions of a court:
wherefore then should thy servant be yet a burden unto my lord the
king?
he could neither take any pleasure himself, nor be of any service to the king, but on the contrary a burden to him; not only on account of his sustenance, and being obliged to have servants to attend him, which might be the least part of his burden, but his company would be disagreeable and troublesome, through the infirmities of old age upon him.