I told my parents once I moved out permanently (at 18), although I was agnostic by 15 or so and an agnostic atheist by 17. Beyond that, I don't blare it out, but I'm comfortable being who I am. If someone raises an issue where it's relevant or if they ask me, I'm quite comfortable being forward and open about it, but I don't go around introducing myself by saying "Hi, I'm Sam, I'm an atheist". Even when newer friends invite me to religious events, I tend to say "Sorry, but I'm not interested in religious events" over "Sorry but I'm an atheist", partly because the former is more true (I've gone to a couple religious events as an atheist observer) and partly because I don't care to fight with a new friend over their beliefs if they're not trying to indoctrinate or convert me.
In this, I do believe in doing unto others as would have them do unto me. I don't want someone shoving their faith system down my throat unless I open debate on a religious issue and so I won't do it to them with my lack of belief.
Right now I'm in a "won't lie if they ask, but I won't come out and spout it" stage. So far, no one's asked. The worst they assume, I'm guessing, is that I'm a backslidden Christian who doesn't go to church. In a way, I am more bothered by that assumption, because it makes me look bad. But since I'm anxious about debating and confrontations (just my personal nature), I avoid bringing things up that will definitely rattle the cages.
I know over time I will be more forthcoming. If I didn't work for a Christian company, I'd be much more public by now.