Well, I started running when I was 13. When things really clicked for me and I took it as something to really seriously pursue, instead of something to just do in my free time, is a time I recall very well.
I was 16 years old, going through the T&F season of my sophomore year. I had earlier run the 1600 and was watching the 800, when I realized that everyone on the team who was better than me was in that race and they were all seniors. Next year, I would be the top returning distance runner for a school with a big reputation and history in distance running and I was only a 5:30 miler/11:30 2 miler. At that moment, I decided that the continuation of the school's distance running tradition was on my shoulders.
The following season, I ran 4:57/10:24 and those times pretty quickly became fairly pedestrian for me. Seemed to work pretty well.