How physically mature is the young athlete. How committed are the young athlete's parents. Learning to compete is just as important as learning to paddle and has been vastly underrated, in my opinion. Physical maturity is not necessary for a beginning slalomist but the earlier a young athlete matures, the earlier that athlete can execute the harder techniques and moves required for top level competition. Jon Lugbill and Martikan are examples of athletes who matured physically at a young age.
Mental maturity is also dependent on individual characteristics.
Some athletes are able to master the highly cognitive aspects of the sport and demonstrate the ability to make correct decisions, solve problems, and execute strategic plans under the pressure of whitewater competition. Davey Hearn and Joe Jacobi are excellent examples of athletes who were cognitive thinkers at a young age. Exposure to paddling, and even slalom, without actually competing, can also affect the optimal age for a young athlete to start the sport. If a youngster has been exposed to paddling it will take less time for that athlete to master the sport than it would for an athlete of similar age who has never been involved in the sport. The willingness of parents to make the necessary sacrifices to get the young athlete to training sessions, races, and training camps will also effect optimal age.