I also got into it through pony club! Pony club is AMAZING, I can't stress it enough. I got out of eventing though when my horse took a bad fall (don't stress though, it was an accident by fault of a clinician I was riding with.. total freak accident.)
What I would suggest for you also, is to volunteer at an event! I know around here, they're always looking for jump judges for cross country (basically you go out with a clipboard and make sure every rider goes over the jump, they don't just trot around it and go on their way.) I jump judged a few times, it is actually a good learning experience to see how the more experienced riders handle their horses on course. You'll also learn a lot just from being around the eventing grounds. And no, you're not too late. I had a short career as an eventer that started at 13 and I quit when I was 16.
Have fun & enjoy!!