I remember going over the "questions" in the Lamp Book.at the age of 12. After all, armageddon was nigh.
Later on, I felt bad because I just got dunked, never having gone to JG in prayer and dedicating my life to do his will. Who knows, maybe this is my loophole? At 16 I recall telling my dad, the PO at the time, that I didn't agree with all the wacky teachings, (the UN, the numerology, the shifting dates, the blood doctrine) but thought it was the "best way of living." Oddly enough that didn't even phase him. I think he agreed. Toward the end of his life, although we never spoke of it, I could see his doubts.
In adulthood, I really grappled with the murder of billions so JG could prove himself right. Serving as an elder, I had a front row seat to the backstabbing, self serving, even criminal behavior of fellow elders. I began to see how my kids were being forced to do something they didn't want and that was pretty much the last straw.
In answer to your question, it was definitely cumulative and took decades. Thanks to the internet, now people can find out immediately that their doubts and concerns are valid. Perhaps this will embolden some to leave more quickly.