The first time I can recall thinking independently was when I was about 10 or 11 years old and I was hanging out in the coat room with some other kids after one of the meetings. This other boy, who was the same age as me said to the rest of us "What would you do if one meeting the Elders all got up on the stage and said 'Ha ha, it was all a joke! Everything we've been telling you all these years was a big joke.'" I think the kid meant it as a joke himself, but I remember thinking to myself that really there is no way to know if everything the elders taught us is true or not except that they say so and my parents and the literature say so.
I wouldn't say that was a real doubt, but I have always recalled it vividly since it made me think. My first solid doubts were when I was in high school and I learned hard science and I could see the proof myself that the human race was not 6,000 years old and there was not a worldwide flood 4,000 years ago that killed everyone except for 8 people and all of the animals in their floating box. I did extra research on my own outside of the school textbooks and read the Creation book and other literature on the subject and I was fairly sure at that point that a hoax had been somehow perpetrated on me, at least with respect to the WTS inventing its own "science". Unfortunately it took me until my mid 20's to realize the full scope of what was going on, but I definitely never argued the literal Genesis account or the flood with anyone out in service.