I'm 31 and have been an atheist since I was about 13 or 14 years old. I was brought up to be a Christian, by my Protestant mother, but my "indoctrination" was paltry at best. The only times I remember my mother taking any of us to a church was a few random times for Christmas Eve services. When I was around six-years-old my parents divorced, and on his weekends to have us my father took us to church. Following my older brothers' example, I ran away to play in the parking lot more often than not. I only have three strong memories of these churches. Strongest is a martial arts exhibition, which if they connected it to the Bible in any way, I cannot now remember. Second is being the new kid at a Sunday school class, and the teacher frowning at me for not knowing the passages the other kids had studied. Last is being given a coloring sheet depicting people worshiping the golden calf. I knew worshiping it was seen as a bad thing, and was at a loss to understand why I was coloring a picture of it. I was not forward enough to ask, plus for the first time in my life I had my grubby little hands on gold and silver crayons, and I intended to use them for all they were worth.
So, there is my Church upbringing: People breaking blocks with their heads is cool, teachers expecting new students to know last week's material are unreasonable, OMFG,THEY MAKE GOLD AND SILVER CRAYONS!!
My mom just told us what she knew of bible stories, god, and Jesus, and simply expected that we would be Christians. We didn't really talk about religion much, it was just something that was. There was no room for questions. I could just as easily have questioned the fact that I was a girl, I was a Christian, and that's the way of it. Her strategy worked well enough it seems, as all four of my siblings are theist. One of them has actually been baptized Catholic. I think if my eldest brother put enough care into thinking about it, and didn't have to worry about how his Catholic wife would receive it, I'm pretty sure he'd be an atheist too. We've only had a few conversations, but I get the impression he leans strongly toward non-belief.
Like I said, though, religion just wasn't something we discussed. In fact it was only a couple months back that my older sister even found out about my atheism. Not that I've been hiding it at all, just that religion matters so little in our family that it had just never come up. I'm pretty sure my mom doesn't know either, because we never have conversations where it would make sense to bring it up, and I don't think it's a big enough deal to make it some sort of pronouncement.
My coworkers do know. I've worked with the same three people for 8-13 years. Two are Catholic, but the one whose religion is strongest is some other type, I don't really know which. They don't treat me differently because of it, except the one has learned to ask me to "keep him/her/ in your thoughts" instead of asking me to pray for someone she wants people to pray for. My boss loves a good conversation about anything, and is open-minded enough to not be offended when his position is questioned. I can't say we've ever had an actual debate, because he's a compulsive liar and will ALWAYS have just spoken to some expert or other on whatever topic we're discussing whose views happen to coincide with his. I don't say that to be disparaging, I truly think he can't help himself and there's no malice in his lies, just constancy.
Like many others here, it was the act of reading the bible that turned me away from belief. Thinking I should strengthen my understanding of what my mom had taught me, I decided to read the stories for myself. I was horrified beyond belief at what the bible actually contained. I stopped believing in god right then, and decided that even if he were real that I could not, and would not follow such a deity.
That was a bit of a ramble, I'm not generally so verbose.
Edited: Because I can't use parentheses.