When I grew up I was taught YECism, although not scientific creationism. I accepted that viewpoint in general. I thought the earth and universe were only a few thousands of years old. I did not, however, think that the six days was to be taken literally. From as early as I can remember I think I thought that God would have done it all instantaneously and that it wouldn't have taken him a day to do each day's work.
The first I remember hearing of the earth's 4.6 billion year age came ag age 8. In a book entitled "The Answer Book of Geography" I remember reading (I think in an article on volcanoes) about the age of the earth and how it might have once been molten. I did not believe it, since the time chronology obviously contradicted that of the Bible.
(Actually I think I might have been exposed to old earth ideas earlier. I had a booklet on dinosaurs from the New York World's Fair. I had been very interested in the dinosaurs when I visited; but I don't think I had ever read the booklets.)
When I was about 10 or 11 I remember reading in a book (The Wonders of Physics--I don't kknow the author) about nucleosynthesis in supernovae and stars. I was impressed that it might be possible to explain the presence of the very chemical elements from simple principles. I think I was very struck by the idea, but it's hard to say whether or not I was convinced. I remember about the same time reading about uranium lead dating. I was able to raise the standard creationist objections (nonconstant half life, initial presence of uranium, nonclosed system), but although not absolutely convincing the basic idea seemed plausible. And there was nothing similarly plausible indicating an earth thousands of years old. And then there was astronomy and its vast distances which would have been meaningless in a thousands-of-years-old earth. I'm pretty sure that biological arguments had little or nothing to do with convincing me. I don't think I had any understanding of natural selection.
In junior high I remember reading about the possibility of a closed but unbounded universe, and that idea fascinated me. But I knew enough to realize that the whole thing wouldn't mean anything if the universe were only thousands of years old.
Although my science edutation was sympathetic to a young earth, we did see movies from the local utility company. Some were about fossils (they were building a nuclear power plant at Calvert Cliffs, where lots of amateurs collected fossils). I think it was in one film that I first heard of the steady state model, where hydrogen was continuously injected into the universe. I think I thought that that was very arbitrary. The Bib Bang, on the other hand, seemed simple, and I was aesthetically drawn to the possibility of a closed, cyclic universe (which would have been the closed, unbounded one I had read of earlier). I think this happened while I was in the ninth grade.
At the same time I began to learn something of Biblical criticism. For Confirmation, as is traditional, I received a Bible of the version of my choice. I chose The Jerusalem Bible because it had a different flavor from most other contemporary translations. (The JB is not derived from the Tyndale/KJV tradition, so it reads somewhat differently.) In any event, the introduction to books and footnotes introduced me to the documentary hypothesis and Biblical criticism generally. I was also learning something of these in my Old Testament class in tenth grade religion. Anyway, I saw concretely that it was possible to understand Genesis and the Bible from a perspective of a community of faith without taking much of it as historical.
That community of faith was important to me. I understood that *I* could hold such interpretation, but then I seemed to myself to be rather different from others, an outlier.
Something else might have been at work. I understood my own faith community (Lutheran) to have been based strongly upon the Bible. Catholics, I thought, would happily ignore or even subordinate the Bible to tradition. But then I saw that Catholic scholarship integrated the Bible consistently with a scientific world view. In my own faith community, in contrast, those views were in tansion, and frequently opposed to each other. It occurred to me that our Biblical scholarship might not be all that it needed to be. That didn't cause a crisis, I think, because at the time there was great conflict between "moderates" and "conservatives"in the Lutheran Church-Missouri Synod. In developing my position in favor of historical criticism (as I learned it was called) I was merely, insofar as my faith was concerned, aligning myself with one church faction.
Strangely enough, tenth grade biology barely mentioned evolution, but did not explicitly promote creationism. I think the only significant discussion of evolution came in my World History class, where the teacher, when lecturing on prehistoric people, mentioned that the story from Genesis wasn't meant to be taken literally. I think that was the first "authority figure" close to me associated with the church who had definitely taken such a position. (I'd heard of others, but didn't know them personally.)
I am sure that I had abandoned creationism by the time I got to college. However, I think that prior to college I thought my (pro-evolution, or at least old earth) viewpoint was a minority viewpoint not only in my own faith community, but among Christians generally. I was pleased to find out that I was mistaken! Both from reading theologians and personal interaction with Christians of many denominations I learned that most of Christianity is quite comfortable with evolution. And I also learned that esseially NONE of science is comfortable with creationism!
AtI also first heard of creation science while an undergraduate, but did not then see any examples of it. It was only when I went to graduate school that I saw issues of the Creation Research Society Quarterly on another graduate student's desk. When I was alone in the office I would sneak over and read articles from an issue. I remember being shocked at their uniformly low quality. I had thought that I might at least be challenbged by some of them, but I remember them to have been uniformly silly. But I was hooked on creationism. It was late in my graduate career when I first read the mini-symposium on changing speed of light and first heard of one Barry Setterfield. It was not until a year or so later, however, when I was doing a post doc at Argonne National Laboratory that I managed find Setterfield's old 1983 article with the famous .99999999+ correlation coeficient for the speed of light data. But that gets way past any possible "conversion" issues on my part.
In summary my mind was changed because an old earth made sense, but a young one didn't. Biology was irrelevant to me. I do not remember any time after about age 11 when I was convinced that creationism was true, although I am not sure that I was convinced it was false. However, while in junior high school I made it a point to learn (and believe) as much of LUtheran Orthodoxy as I could. I think an important source book was Koehler's "A Summary of Christian Doctrine", which, as were most Lutheran Church-Missouri Synod publications, was entirely YEC. So I must have held some contradictory notions, but I am not sure how I tried to resolve them.
And of course these are my memories without documentary support. I think you should be suspicious of them. It seems to me that what I remember myself thinking casts me in too "heroic" a light. I appear too rational for a child or adolescent. I suspect that there were complecations, dubts, and regressions,; but I cannot clearly remember them.