When I was a young child my patents were Christians, we went to church with their friends, and we went to church kids groups, and later a Lutheran school. I am also fairly sure that I have been baptised. That being said, I can't say exactly how old I was, because I feel I was simply too young to remember.
I do remember, however, that in early grades of primary school, I got in trouble for telling kids at school that god isn't real. In kindergarten, I was thinking with the mind of a teenager, eg, we made ttelevisions out if carboard boxes, and on each of my channels was a different of our teachers (the hot ones) dancing naked. Highly inappropriate for a 4/5 year old, but I had also already learned how to manipulate situations to get my way and lie to get what I want.
I wish I could say I never believed in god, or the idea of God's at all in my own case, but I know that I must at some stage have believed it, simply because it was taught to me since birth. But it is definite in my memory that I did not believe in God from the age of 10.