Depends on the 5 year old I'd say, if they've been brought up to believe in God, their faith is generally unshakable. Their world (hopefully) consists of the love of their parents and that is pretty much it. Thus I think a five year old has much more experience of omnipotence, omniscience and love because that is how their parents (hopefully, again) have been seen by that 5 year old. Also, they are not confused or led astray by conflicting denominations or any potential contradictions in scripture because they have no knowledge of them.
An old man would need to have resolved all of these issues to know God, which is a difficult thing to do. They are constantly required to judge if something is the word of God or a mutation of it brought about by man since the writings were first written.
The 5 year old does not need scripture to know God, they are as free of inner conflict and as familiar with the concepts associated with God as can be.
So my money is on the 5 year old.
Don't let me down little Jimmy.