I gave up believing in God and Santa at about the same time. When it was explained to me that Santa was just a pretty story to make Christmas feel a bit more special, it not long afterwards occurred to me that quite a lot of things I had believed up till then seemed awfully convenient with hindsight.
Having worked out that my presents came from the shops, and that my parents had to pay for them, it was on reflection ludicrous to then assume that they had them delivered to the North Pole so that a man with a sack and magic, flying reindeer could bring them to my house and sneak down our chimney to give them back. In a simple, intuitive way I had arrived at a version of Ockham's razor, and having done so I proceeded to apply it to all the things I had previously taken for granted. At which point global floods, talking snakes, angels, devils, virgin births, men rising from the dead and miracles didn't make the cut. If I could think of a simpler explanation, the more complex one was rejected.
So for me it was finding out that there isn't a Santa Claus that was the trigger. It's only a shame that that first step didn't come until I was 23.