It may not be a certain age, it can be a traumatic event, that helps start thinking.
For me, it was different, I always wanted to look at facts, avoid being close minded. However, most research i did, was in WT -library...still I found somethings always questionable, like blaming 1975 fiasco on the brothers, or believing that tigers, sharks etc in paradise will eat straw...
In my case, it was this open mindset, that when I stumbled across facts regarding blood policy I just soaked all the information up like a dry spounge, the beginning of a fast, depressing but also exciting road, to where I stand now after three years as an agnostic, who formally is still in because of family....
I remember, I always was curious for information, even pre internet, I looked secretly in a book written by a former member (very popular in my area, many book stores displayed it in the early nineties) but he wrote lies and half-truths, which confirmed by belief in the org, rather than destroyed it.
Well, but if you really appreciate facts, sooner or later, the time comes and they will shine through myth. Point is, a person has to be ready to accept it even if it may go against wishful thinking...