I have to agree with @sirveloman.
I raced NICA league through high school and loved it, and in my experience, it’s easy to overdo it too early. The defining factor between an athlete burning out versus being successful boils down to who is putting pressure on the young athlete. If the athlete is putting pressure on themselves, then structured training is a great tool. If coaches or parents are putting pressure on them to train, even with the very best of intentions, then the outcome tends to be less favorable.
A little background about me, I started mountain biking in 8th grade and just trained for fun with my high school through my sophomore year of high school. At this point, I never really felt like I was “training”, I was just having fun riding with my friends. By “just having fun” I got plenty fast and did very well in JV as a Sophmore. Since I was moving up to Varsity my Junior year, I ramped up my training and started training alone and with more focus. I had my best year to date, winning the overall Northern Division and taking Second at State Championships. That year helped me make make the roster for Bear Development Team, but it also burned me out mentally. I no longer had the community around me that made training seem fun and rewarding. Nurturing that community and passion for the sport will result in much more significant gains than starting structured training too early. Happy racers will always go faster.
In my opinion, when your young athlete comes to you and expresses a desire to get faster and wants to be more competitive, that is a great time to start introducing structure and gradually teaching him proper training techniques.
I hope me sharing my experience has been helpful for you!