When do you teach children about God?

Ideally, I'd like to start teaching them as young as possible but I would also want them to be old enough to grasp concepts and understand the purpose of religion. I say this because my religious education started really young and I remember going to religious classes on Sundays and having alot of questions but was chided by my teacher. I guess back in the day (or maybe it was just my teacher), religion works on a strictly-just-follow-or-else-get-punished policy. From my friends who do send their kids for classes, I understand that religious classes are nothing like what it was back when I was a kid, which I am glad.