When do you teach children about God?

We are not a religious household at all, so for daughter God was not something that she took for granted. She started asking about God -- because other adults around her spoke of it -- when she was just two years old. I think kids that age are able to understand that their is a being called "God" who we pray to. The questions become more complex as they get older. My daughter is now 6.5 and asks about why people have different gods, does god punish, why does he punish, can you see god, who decides which person gets which god -- these questions are harder to answer, especially if parents are not religious. However, exposing your child to religion is very different from talking to your child about God.