I don't think I can point to a certain time in my life and say: "That's when I stopped believing".
I guess if I ever truly believed in anything, it was who we call "christkind" (might translate to "baby jesus", and takes on Santa Claus' function in Germany), and similar "anthropomorphic personifications", like the Easter Bunny or Saint Niclas.
Now that I think about it, these are probably the only "imaginary" (everything imaginable is real in some way

) people I ever truly believed in in the way society would have expected me to. I even scattered carrots around our house each Easter Eve.
Besides them, I always firmly believed in science. I know that lots of people say you can't actually believe in scientific things because they can be proven, but I did anyway. My "bible" was a tinkering book with relatively harmless experiments in it, and my electronic circuitry set was a kind of altar to me, to stress the metaphor a little ^^
I guess I did believe in god though, and in a way I still do, and I feel sorry for him, really. It's not nice what the church did to him.
I believed that my cuddlytoys came to life at night to protect me. Maybe they did, I don't know.
And today I see Optimus Prime as a kind of religious icon ^^
Really, I think he's one of the most morally superior fictional characters ever created.