Not to dissuade further discussion, but this question has already been answered multiple times:
http://www.reddit.com/r/AskHistorians/comments/21igap/the_christian_god_is_often_depicted_as_an_older/
http://www.reddit.com/r/AskHistorians/comments/z6669/how_did_the_christian_god_come_to_be_visualized/
http://www.reddit.com/r/AskHistorians/comments/1ps4nj/why_did_michelangelo_paint_god_as_an_old_man_with/