Why did the prophets use polytheistic description of God. For e.g. Daniel described God as old man (Dan. 7:9) which might be borrowed by appearance of Greek God Zeus?

Please forgive me, but I got a bit of a laugh from this question.  You are misunderstanding Daniel 7:9 (and Revelation 1:14).  The whiteness of the hair is not a sign of an “old man”!!!  That is for sure.  Please look at the parallel verse in Revelation 1:14 in which we have an image of Jesus with hair as white as snow, eyes like blazing fire, feet like bronze and a voice like the sound of rushing waters.  I am sure that the actual vision that John saw was even more awesome (and I mean literally awesome) than he can describe with his metaphors.  You can see from this vision that this is no doddering old man.  This is a massively, incredibly, overwhelmingly powerful figure, whose hair is white, not because he is old, but because he is pure and holy.

To be fair, I believe you are probably even misjudging Greek ideas about Zeus!   He may appear “old” in Greek mythology, but that is an age of wisdom,  not of weakness, and his hair is white (even though he is a myth) because of his holiness as well.  This picture of a weak, tired, old God is the product of Western anti-God sentiment, certainly not because of anything in the Bible, or even in Greek mythology. Of course, although the Greeks may describe Zeus as old to represent his wisdom, the God of the Bible is immortal, unchanging and ageless.  He is in no way “old.”

So, I suggest you change your mental image from Daniel 7:9 to match that you get from Revelation 1:14.

John Oakes