I actually did have a defining moment in my life, though it did not result in an instantaneous transformation into being a skeptic.
I was raised in a quasi-catholic family and attended a parochial school from grades 2 through 6. At the time I believed pretty much all the silly stuff that they threw at me, because hey, they were adults and they obviously knew what they were talking about. I also had a fledgling interest in things such as ESP and UFOs, because as another poster said, it just seemed so cool.
My defining moment happened during a 5th grade science class where we were learning about the solar system. The teacher, Mrs. Rousi (interestingly, my brain is still devoting a portion of its efforts to storing her name over 30 years later) drew the standard picture of the sun, Mercury, Venus, Earth, etc. on the blackboard and told how the planets all circled around the sun. Yes, this was a progressive Catholic school that actually accepted that Earth was not at the center.
Anyway, I raised my hand and asked a rather simple question: What keeps the planets in their orbits? Why don't they just fly off?
I really don't think a quick explanation of gravitational attraction would have been too much of a reach for a room full of 5th graders, but Mrs. Rousi either didn't want to go that route or maybe she didn't know the answer. But without missing a beat, she replied that it was because, "that's just the way God made it."
I immediately thought, "Hey, wait a minute... she doesn't know the answer. She's making this up!" As I said, I did not instantly become a full-blooded skeptic at the moment, but that little exchange got me thinking that maybe, just maybe, some of what the adults were saying wasn't true.
By my early teens I had become very uncomfortable with religion, but was still holding on to some of the other woos, primarily ESP. Again, this was primarily because I thought it would be so freakin' cool if it worked. I really can't pinpoint the moment when I realized that ESP and the likes were baseless, but I'd say that by the time I got to college I was pretty well entrenched in my skeptical ways, though I didn't know there was a name for it.