After my first ever coaster ride on Loch Ness Monster, I was actually pretty afraid of coasters for a couple years. I would only be able to work up the courage to ride one or two per day if my family went to a theme park. Despite that, I was fascinated by them and would spend hours learning about them online, looking at pictures on RCDB, reading articles, and watching videos.
In 1999, Apollo's Chariot started to lighten my coaster fears. Sure, my first ride on it and the next few after that were pretty scary to me, but so thrilling and fun at the same time that I didn't mind getting back in line. It even helped me convince myself I could ride Alpengeist, which I had never ridden in the 2 years previous that I had been going to Busch Gardens!
The first time I can remember thinking like an enthusiast, though, was my first trip to Busch Gardens Tampa back in 2003. I had been scouting Kumba & Montu on RCDB and was super excited to ride them. This is the earliest I can remember riding a coaster for the first time without a hint of fear or intimidation, just pure excitement. So I guess I've been an "enthusiast," in some sense of the term, for about 9 years. But as far as the keeping-track-of-credits and planning-trips-based-entirely-around-theme-parks aspects go, that's only been the case for me since 2007, my first trip to Cedar Point.