I was a firm believer in YEC up to the time I was an undergraduate sophomore. On a seventh grade history quiz I prefaced every sentence answering questions about prehistoric man with "According to my history book...". I argued with my high school biology teacher for the biblical creation account and against evolution. As a college freshman I wrote a paragraph in a psychology paper trying to convince my professor that he should believe in God (he was an agnostic). I got annoyed with an upperclassman who pointed out to me all of the practical problems associated with the biblical flood. It was my understanding of the ToE and the evidence for an OE as an adult that changed my mind. I hung on to the creation and flood story (brief flirtation with OE creationism and "kinds" hyperevolution after the flood) a little longer but finally I let that go also.
Paul in I Corinthians 13 Verse 11 writes:
When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things.