Mine was the Spider at Lagoon when I was 6
I was terrified the whole time, and when I got off, I said, "Well, I didn't die."
My dad gave me $5 and I didn't ride any other coasters that day.
My other early coasters were Space Mountain at DisneyWorld (my mom told me it was for 7-year-old boys and I believed her... the whole ride I was screaming "THIS RIDE IS NOT FOR SEVEN-YEAR-OLD BOYS!!!!!") and Big Thunder Mountain Railroad (same day as Space Mountain. My dad told me that he didn't like Space Mountain either because he felt like he was going to hit his head, so when everyone else went to Splash Mountain, we went to this one. It was pretty fun but I was really irritated waiting in line and I wanted to leave.
My opinions on roller coasters really have changed since then, haha