There is some truth here, @Patrick, I’m afraid. I don’t have statistics to cite, but I know many people who started in the YEC camp and ended up as total unbelievers. Those are private stories that I won’t relate, but some of them are well known public stories, including my longtime friend Ronald Numbers (Ronald Numbers - Wikipedia). At the beginning of his book, The Creationists, Ron tells the story of a personal faith journey from being a Seventh-day Adventist (that’s where the YEC view came from) to being an agnostic, starting with the realization that “the earth was at least thirty thousand years old,” a conclusion he drew as a graduate student in history when he attended a lecture on the fossilized forests in the Yellowstone River Canyon. “I quickly, though not painlessly, slid down the proverbial slipperly slope toward unbelief.”
This happens. A lot. Are Ken Ham and company responsible? IMO, yes, to some extent–to the extent that they try to persuade Christians that no alternative understanding of early Genesis is acceptable, that everything else is the beginning of apostacy.
At the same time, there are lots of people who start somewhere else–even with a TE position–who also end up in unbelief, for various reasons. It’s complicated: each person’s story is unique. And, of course, there are people who go from unbelief to belief, for various reasons, including Francis Collins and C. S. Lewis and many others. In no case that I am personally aware of, however, did conversion from unbelief to belief involve someone being persuaded that the earth is “young.” Lots and lots of people have lost their faith after becoming persuade that the earth is “old.”