The first time I had doubts was when I was about 10 or 11. We were studying the Revelation book and were told that several prophecies had found their accomplishment in events that occurred with the Watchtower. Even at that age, I felt it was ridiculous.
I asked my dad and he told me that made sense to him and then, proceeded to basically read the same study all over.
I asked my mom and she told me to ignore these. That weak JWs around us had placed to much importance in the prophecies and left the truth entirely; missing the essential part of it: Preaching work and to stay separate from the wicked world.
Then, during meetings, I remember being told: If you have questions, you need to do more research. We have plenty of documentation where you can find ALL the answers you need.
So, until the age of 25, I blamed myself for not setting enough time aside to research my personal questions. When I finally did those EXTENSIVE researches (I once calculated about 800 hrs in a year), I fell back on my mom's reasoning: The main important thing is the preaching work and staying separate from the wicked world.
It was only around the age of 35 that I finally realised: What good is the preaching work if what is thought is wrong? You can translate an error in hundreds of languages, print it in millions of magazines and books, distributing them all over the planet, and it will still be untrue.
An organisation that gives out fallacious and deceptive information and answers to very valid questions cannot be called "the truth".
As for staying separate from the wicked world, I realised that the JWs had just as much serious issues than anyone else on the planet! Divorce, drugs, suicides, depression, bad business practices, etc. Sure, they disfellowship the wrong doers! Still, the evil deed was done by an active JW at the time! For instance, if a man is disfellowshiped for cheating on his wife, the fact remain that it was a JW who cheated on his JW wife. It happened while both were in the "spiritual paradise". If such a thing was a rare occurrence, than, perhaps it could be dismissed. Unfortunately, when you're a JW long enough, you get to see that it is much more prevalent then the watchtower would ever tell you.