The point of Michelangelo's art was not to create an object of worship but to communicate truth, and in this case, it is the story of creation. That fresco on the ceiling of the Sistine Chapel shows how the human soul is a special creation that comes directly from God. It also shows the tender love God has for Adam and thus for all of us.

I don't think that Michelangelo was trying to tell us that God looks like an old man. Instead, the old man image is a way to visually represent the description of God the Father as "the Ancient of Days" from Daniel 7:13, "... there came one like a son of man, and he came to the Ancient of Days and was presented before him."